diff --git a/.claude/rules/stack-and-commands.md b/.claude/rules/stack-and-commands.md
index 6ffaaad..7ae9309 100644
--- a/.claude/rules/stack-and-commands.md
+++ b/.claude/rules/stack-and-commands.md
@@ -39,6 +39,8 @@ Sandbox CLI (v0):
 sandboxer create                              # canary self-deploy (profile.sandbox-canary)
 sandboxer create --profile profile.compose-e2e --input repo=/path/to/repo
 sandboxer create --profile profile.vm-haskell-build --input vm=haskell-build --input repo=/path
+sandboxer create --profile profile.saas-stub    # metered SaaS stub
+sandboxer credits show / credits add 10.00
 sandboxer get <id>
 sandboxer list
 sandboxer destroy <id>
diff --git a/INTENT.md b/INTENT.md
index 2c21072..d56dd07 100644
--- a/INTENT.md
+++ b/INTENT.md
@@ -357,6 +357,7 @@ follows evidence.
 5. ~~**Registry entry**~~ — `capability.execution.sandbox-provision`
 6. ~~**Sibling integration notes**~~ — `docs/integrations/`
 7. ~~**Extension SDK sketch**~~ — done (`docs/extension-sdk.md`, `ext.vm-packer` attach mode)
+9. ~~**SaaS extensions + payments v0**~~ — done (`ext.saas-stub`, routing, credits — SAND-WP-0006)
 8. ~~**wise-validator**~~ — sibling repo (SAND-WP-0003); not a sand-boxer dependency
 
 ---
diff --git a/SCOPE.md b/SCOPE.md
index 193fb26..5b03fdb 100644
--- a/SCOPE.md
+++ b/SCOPE.md
@@ -107,7 +107,8 @@ own tunnels or CAs.
 - **Workplans finished:** SAND-WP-0001–0005, 0008 (see `workplans/`)
 - **Package:** `src/sandboxer/` — CLI, manager, extensions, telemetry, HTTP API
 - **Profiles:** `profile.compose-e2e`, `profile.sandbox-canary`, `profile.vm-haskell-build`
-- **Extensions:** `ext.compose-ssh`, `ext.vm-packer` (attach mode)
+- **Extensions:** `ext.compose-ssh`, `ext.vm-packer`, `ext.saas-stub` (metered)
+- **Routing + payments:** `docs/routing.md`, `docs/payments.md`, `sandboxer credits`
 - **Registry:** `capability.execution.sandbox-provision` indexed (draft)
 - **Tests:** 26 pytest cases; `make check` green
 - **Sibling:** wise-validator ships `validate run` (SAND-WP-0003)
@@ -144,7 +145,7 @@ cd ~/the-custodian && make e2e REPO=activity-core
 - ~~`make e2e REPO=` shim~~ — done (SAND-WP-0004; delegates to `validate run`)
 - TTL auto-expiry / `extend_ttl` enforcement
 - ~~`ext.vm-packer` attach mode~~ — done (SAND-WP-0005); Packer build orchestration deferred
-- SaaS extensions (E2B, Modal) or payments layer (SAND-WP-0006)
+- Real E2B / Modal adapters (stub + payments v0 done in SAND-WP-0006)
 - Snapshot / restore / checkpoint profiles (SAND-WP-0007)
 - Formal ops-bridge tunnel attachment in reachability descriptor
 - Dedicated sandboxer01 host (CoulombCore interim only today)
diff --git a/docs/extension-sdk.md b/docs/extension-sdk.md
index c1c6319..2a4a90f 100644
--- a/docs/extension-sdk.md
+++ b/docs/extension-sdk.md
@@ -92,11 +92,16 @@ with patch.object(SSHConfig, "run", return_value=(0, "ready")):
     handle = ext.provision(profile, {"vm": "haskell-build"}, "localhost")
 ```
 
+## Metered extensions (SAND-WP-0006)
+
+Implement `estimate_cost` and `meter_actual` on `SandboxExtension`. Register with
+`pricing_model: metered`. See `docs/payments.md` and `ext.saas-stub`.
+
 ## Deferred
 
 | Feature | Workplan |
 |---------|----------|
 | Packer build orchestration from `create` | Future WP |
-| SaaS adapters + `estimate_cost` | SAND-WP-0006 |
-| Multi-backend routing engine | SAND-WP-0006 |
+| E2B / Modal / Daytona cloud adapters | Post SAND-WP-0006 |
+| fin-hub billing export | Future |
 | Snapshot / restore hooks | SAND-WP-0007 |
\ No newline at end of file
diff --git a/docs/meta-framework.md b/docs/meta-framework.md
index 702b2fc..a323c67 100644
--- a/docs/meta-framework.md
+++ b/docs/meta-framework.md
@@ -153,7 +153,7 @@ When multiple extensions satisfy a profile capability:
 | `lowest-latency` | Closest region / host wins |
 | `explicit` | Profile names a single `extension`; no auto-routing |
 
-v0 resolves `profile.extension` directly — routing engine deferred to SAND-WP-0006.
+Routing engine v0: `sandboxer.routing.resolver` — see `docs/routing.md`.
 
 ---
 
diff --git a/docs/migration-gaps.md b/docs/migration-gaps.md
index a014ba9..ea4be12 100644
--- a/docs/migration-gaps.md
+++ b/docs/migration-gaps.md
@@ -43,6 +43,7 @@ Deferred: Packer orchestration from API, `make remote-build` shim.
 
 | Item | Workplan |
 |------|----------|
-| SaaS extensions + payments | SAND-WP-0006 |
+| ~~SaaS extensions + payments v0~~ | SAND-WP-0006 — stub + routing + credits |
+| E2B / Modal real adapters | Post SAND-WP-0006 |
 | Snapshot / restore | SAND-WP-0007 |
 | TTL enforcement + scheduled reap | TBD |
\ No newline at end of file
diff --git a/docs/payments.md b/docs/payments.md
new file mode 100644
index 0000000..ec137c7
--- /dev/null
+++ b/docs/payments.md
@@ -0,0 +1,45 @@
+# Payments and metering
+
+Version 0.1 — SAND-WP-0006. OpenRouter-style credits for metered SaaS extensions.
+
+## Credits store
+
+Path: `~/.local/share/sandboxer/credits.json` (override via test injection).
+
+| Env | Default |
+|-----|---------|
+| `SANDBOXER_DEFAULT_CREDITS` | `10.0` USD on first use |
+
+```bash
+sandboxer credits show
+sandboxer credits add 25.00
+```
+
+## Metered lifecycle
+
+1. **create** — `estimate_cost` on metered extension; block if balance insufficient
+2. **ready** — `SandboxStatus.meter.estimate_usd` recorded
+3. **destroy** — `meter_actual` (or prorated estimate); debit credits; State Hub note event
+
+Self-hosted extensions (`pricing_model: self-hosted`) skip credits.
+
+## Extension contract
+
+Metered extensions implement on `SandboxExtension`:
+
+```python
+def estimate_cost(self, profile, inputs, *, duration_s=3600) -> MeterQuote | None
+def meter_actual(self, handle, *, duration_s: float) -> float | None
+```
+
+Reference: `ext.saas-stub` (no external API).
+
+## BYOK
+
+Provider API keys are resolved at provision boundary via `secret_refs` / OpenBao —
+not implemented in v0 stub. Set provider env vars per extension when adapters land.
+
+## Billing export
+
+sand-boxer meters sandbox consumption only. Domain billing authority (fin-hub) is a
+future export consumer of State Hub meter events — not owned here.
\ No newline at end of file
diff --git a/docs/routing.md b/docs/routing.md
new file mode 100644
index 0000000..e26c028
--- /dev/null
+++ b/docs/routing.md
@@ -0,0 +1,44 @@
+# Extension routing
+
+Version 0.1 — SAND-WP-0006. Select backend when a profile lists multiple extensions.
+
+## Profile route block
+
+```yaml
+extension: ext.compose-ssh   # default / explicit fallback
+route:
+  strategy: prefer-self-hosted
+  extensions:
+    - ext.compose-ssh
+    - ext.saas-stub
+  max_cost_per_hour_usd: 1.0
+```
+
+## Strategies
+
+| Strategy | Behavior |
+|----------|----------|
+| `explicit` | Use `profile.extension` (default when no route) |
+| `prefer-self-hosted` | First self-hosted candidate with resolvable host; else SaaS |
+| `lowest-cost` | Self-hosted if available; else cheapest `estimate_cost` |
+| `lowest-latency` | Self-hosted if available; else last candidate (v0) |
+
+## Testing SaaS fallback
+
+```bash
+# Skip self-hosted when host unavailable:
+unset SANDBOXER_HOST
+SANDBOXER_FORCE_SAAS=1 sandboxer create --profile profile.burst-sandbox
+
+# Or use metered-only profile:
+sandboxer create --profile profile.saas-stub
+```
+
+## Reference profiles
+
+| Profile | Route |
+|---------|-------|
+| `profile.burst-sandbox` | compose-ssh → saas-stub fallback |
+| `profile.saas-stub` | explicit `ext.saas-stub` |
+
+Resolver: `sandboxer.routing.resolver.resolve_extension`.
\ No newline at end of file
diff --git a/extensions/README.md b/extensions/README.md
index 778d302..fb2f123 100644
--- a/extensions/README.md
+++ b/extensions/README.md
@@ -46,4 +46,14 @@ Attach mode for pre-built VMs (`the-custodian/infra/build-machines/` lineage).
 
 **Profile:** `profile.vm-haskell-build` — see `docs/runbooks/profile-vm-haskell-build.md`.
 
-Packer build / OVA import remains operator-driven (not triggered by `create`).
\ No newline at end of file
+Packer build / OVA import remains operator-driven (not triggered by `create`).
+
+## ext.saas-stub
+
+Metered SaaS stub for payments and routing v0 (SAND-WP-0006). No external API.
+
+**estimate_cost / meter_actual:** credits check on create; debit on destroy.
+
+**Profile:** `profile.saas-stub` (explicit), `profile.burst-sandbox` (self-hosted fallback).
+
+See `docs/payments.md` and `docs/routing.md`.
\ No newline at end of file
diff --git a/extensions/ext.saas-stub.yaml b/extensions/ext.saas-stub.yaml
new file mode 100644
index 0000000..0fbd2b3
--- /dev/null
+++ b/extensions/ext.saas-stub.yaml
@@ -0,0 +1,15 @@
+id: ext.saas-stub
+title: SaaS sandbox stub
+description: >
+  Metered SaaS extension stub for payments and routing v0. No external API;
+  use for burst fallback testing before E2B/Modal adapters land.
+handler: sandboxer.extensions.saas_stub:SaaSStubExtension
+capabilities:
+  isolation_levels: [microvm]
+  regions: [us, eu]
+  persistence: true
+  pricing_model: metered
+config:
+  provider: saas-stub
+  rate_usd_per_hour: 0.12
+  session_fee_usd: 0.01
\ No newline at end of file
diff --git a/profiles/profile.burst-sandbox.yaml b/profiles/profile.burst-sandbox.yaml
new file mode 100644
index 0000000..a269ef8
--- /dev/null
+++ b/profiles/profile.burst-sandbox.yaml
@@ -0,0 +1,40 @@
+id: profile.burst-sandbox
+version: "1.0.0"
+extension: ext.compose-ssh
+route:
+  strategy: prefer-self-hosted
+  extensions:
+    - ext.compose-ssh
+    - ext.saas-stub
+  max_cost_per_hour_usd: 1.0
+isolation:
+  level: container
+network:
+  default: deny
+  egress: []
+workspace:
+  mode: remote-canonical
+  access: rw
+scope_default: session
+ttl:
+  default: 2h
+  max: 8h
+  idle_reap: null
+resources:
+  cpu: null
+  memory_mb: null
+setup:
+  instructions: >
+    Prefer self-hosted compose on SANDBOXER_HOST; falls back to metered SaaS stub
+    when host is unavailable or SANDBOXER_FORCE_SAAS=1.
+  secret_refs: []
+placement:
+  prefer: [sandboxer01]
+  fallback: [coulombcore]
+reachability:
+  tunnel: ops-bridge
+  identity: ops-warden
+metadata:
+  cost_class: saas-metered
+  latency_class: standard
+  observability: none
\ No newline at end of file
diff --git a/profiles/profile.saas-stub.yaml b/profiles/profile.saas-stub.yaml
new file mode 100644
index 0000000..6946d07
--- /dev/null
+++ b/profiles/profile.saas-stub.yaml
@@ -0,0 +1,32 @@
+id: profile.saas-stub
+version: "1.0.0"
+extension: ext.saas-stub
+isolation:
+  level: microvm
+network:
+  default: deny
+  egress: []
+workspace:
+  mode: remote-canonical
+  access: rw
+scope_default: session
+ttl:
+  default: 1h
+  max: 4h
+  idle_reap: null
+resources:
+  cpu: null
+  memory_mb: null
+setup:
+  instructions: "Metered SaaS stub — no external provider API."
+  secret_refs: []
+placement:
+  prefer: []
+  fallback: []
+reachability:
+  tunnel: ops-bridge
+  identity: ops-warden
+metadata:
+  cost_class: saas-metered
+  latency_class: standard
+  observability: none
\ No newline at end of file
diff --git a/registry/capabilities/execution.sandbox-provision.md b/registry/capabilities/execution.sandbox-provision.md
index 1f0815d..cab453d 100644
--- a/registry/capabilities/execution.sandbox-provision.md
+++ b/registry/capabilities/execution.sandbox-provision.md
@@ -33,7 +33,7 @@ external_evidence:
       - profile-based create/destroy via CLI
       - State Hub lifecycle events on transitions
     broken_expectations:
-      - SaaS extensions and payments layer not yet built
+      - Real E2B/Modal adapters not yet built (saas-stub + credits v0 done)
       - wise-validator migration not complete
     out_of_scope_expectations:
       - agent harness and tool orchestration (glas-harness)
diff --git a/src/sandboxer/cli.py b/src/sandboxer/cli.py
index 9780513..f4031b8 100644
--- a/src/sandboxer/cli.py
+++ b/src/sandboxer/cli.py
@@ -11,6 +11,7 @@ from sandboxer import __version__
 from sandboxer.core.manager import SandboxManager
 from sandboxer.defaults import resolve_create_defaults
 from sandboxer.models import ActorType, Consumer, SandboxCreateRequest
+from sandboxer.payments.credits import CreditsStore
 from sandboxer.placement import resolve_host
 from sandboxer.profiles.loader import load_profile
 from sandboxer.telemetry.export import export_telemetry
@@ -25,6 +26,8 @@ app = typer.Typer(
 )
 inspect_app = typer.Typer(help="Host introspection without provisioning.")
 app.add_typer(inspect_app, name="inspect")
+credits_app = typer.Typer(help="SaaS sandbox credits (metered extensions).")
+app.add_typer(credits_app, name="credits")
 
 
 @app.callback()
@@ -216,5 +219,22 @@ def reap_stale_cmd(
     _print_json([r.model_dump(mode="json") for r in results])
 
 
+@credits_app.command("show")
+def credits_show() -> None:
+    """Show current credit balance."""
+    store = CreditsStore()
+    _print_json({"balance_usd": store.balance(), "currency": "USD"})
+
+
+@credits_app.command("add")
+def credits_add(
+    amount: Annotated[float, typer.Argument(help="USD amount to add")],
+) -> None:
+    """Add credits to the workspace balance."""
+    store = CreditsStore()
+    new_balance = store.add(amount)
+    typer.echo(f"Balance: {new_balance:.4f} USD")
+
+
 if __name__ == "__main__":
     app()
\ No newline at end of file
diff --git a/src/sandboxer/core/manager.py b/src/sandboxer/core/manager.py
index ffef014..502e88f 100644
--- a/src/sandboxer/core/manager.py
+++ b/src/sandboxer/core/manager.py
@@ -6,13 +6,17 @@ from sandboxer.extensions.registry import load_extension, resolve_backend
 from sandboxer.lifecycle.state_hub import emit_lifecycle_event, event_type_for_state
 from sandboxer.lifecycle.store import SandboxStore, utcnow
 from sandboxer.models import (
+    MeterRecord,
     Reachability,
     SandboxCreateRequest,
     SandboxState,
     SandboxStatus,
 )
+from sandboxer.payments.credits import CreditsStore
+from sandboxer.payments.metering import estimate_cost, settle_usage
 from sandboxer.placement import resolve_host
 from sandboxer.profiles.loader import load_profile
+from sandboxer.routing.resolver import resolve_extension
 from sandboxer.telemetry.export import export_telemetry
 from sandboxer.telemetry.introspection import (
     build_introspection_report,
@@ -22,17 +26,40 @@ from sandboxer.telemetry.introspection import (
 
 
 class SandboxManager:
-    def __init__(self, store: SandboxStore | None = None) -> None:
+    def __init__(
+        self,
+        store: SandboxStore | None = None,
+        credits: CreditsStore | None = None,
+    ) -> None:
         self.store = store or SandboxStore()
+        self.credits = credits or CreditsStore()
+
+    def _resolved_host(self, profile, extension, host_override: str | None) -> str:
+        if extension.capabilities.pricing_model == "metered":
+            return extension.config.get("provider", "saas")
+        return resolve_host(profile, override=host_override)
 
     def create(self, request: SandboxCreateRequest, *, host: str | None = None) -> SandboxStatus:
         profile = load_profile(request.profile)
-        extension = load_extension(profile.extension)
+        extension = resolve_extension(profile, request.inputs, host_override=host)
         backend = resolve_backend(extension)
-        resolved_host = resolve_host(profile, override=host)
+        resolved_host = self._resolved_host(profile, extension, host)
         wants_telemetry = profile_wants_telemetry(profile)
         base_dir = extension.config.get("base_dir", "/tmp/sandboxer")
 
+        quote = estimate_cost(extension, profile, request.inputs)
+        meter_record: MeterRecord | None = None
+        if quote:
+            if not self.credits.can_afford(quote.estimated_usd):
+                raise RuntimeError(
+                    f"Insufficient credits: need {quote.estimated_usd:.4f} USD, "
+                    f"balance {self.credits.balance():.4f} USD"
+                )
+            meter_record = MeterRecord(
+                pricing_model="metered",
+                estimate_usd=quote.estimated_usd,
+            )
+
         now = utcnow()
         status = SandboxStatus(
             sandbox_id="pending",
@@ -42,6 +69,7 @@ class SandboxManager:
             consumer=request.consumer,
             host=resolved_host,
             inputs=dict(request.inputs),
+            meter=meter_record,
             created_at=now,
             updated_at=now,
         )
@@ -52,7 +80,7 @@ class SandboxManager:
         emit_lifecycle_event(status, event_type=event_type_for_state(status.state))
 
         provision_before = None
-        if wants_telemetry:
+        if wants_telemetry and extension.capabilities.pricing_model != "metered":
             provision_before = collect_host_snapshot(resolved_host)
 
         try:
@@ -64,6 +92,7 @@ class SandboxManager:
             status.inputs["ssh_port"] = handle.get("ssh_port", "")
             status.inputs["vm_target"] = handle.get("vm_target", "")
             status.inputs["vm_host"] = handle.get("vm_host", "")
+            status.inputs["endpoint"] = handle.get("endpoint", "")
             reach = backend.wait_ready(handle)
             status.reachability = Reachability(**reach)
             status.state = SandboxState.READY
@@ -114,13 +143,13 @@ class SandboxManager:
             return status
 
         profile = load_profile(status.profile_id)
-        extension = load_extension(profile.extension)
+        extension = load_extension(status.extension_id)
         backend = resolve_backend(extension)
         wants_telemetry = profile_wants_telemetry(profile)
         base_dir = extension.config.get("base_dir", "/tmp/sandboxer")
 
         destroy_before = None
-        if wants_telemetry and status.host:
+        if wants_telemetry and status.host and extension.capabilities.pricing_model != "metered":
             destroy_before = collect_host_snapshot(status.host)
 
         status.state = SandboxState.DESTROYING
@@ -139,6 +168,7 @@ class SandboxManager:
             "ssh_port": status.inputs.get("ssh_port", ""),
             "vm_target": status.inputs.get("vm_target", ""),
             "vm_host": status.inputs.get("vm_host", ""),
+            "endpoint": status.inputs.get("endpoint", ""),
         }
         backend.teardown(handle)
 
@@ -146,6 +176,19 @@ class SandboxManager:
         status.destroyed_at = utcnow()
         status.updated_at = status.destroyed_at
 
+        settled = settle_usage(status, extension, handle, destroyed_at=status.destroyed_at)
+        if settled and settled.pricing_model == "metered" and settled.actual_usd:
+            self.credits.debit(settled.actual_usd)
+            status.meter = settled
+            emit_lifecycle_event(
+                status,
+                summary=(
+                    f"Sandbox metered: {settled.actual_usd:.4f} USD "
+                    f"({settled.duration_s:.0f}s, ext={extension.id})"
+                ),
+                event_type="note",
+            )
+
         if wants_telemetry and destroy_before and status.host:
             destroy_after = collect_host_snapshot(status.host)
             report = build_introspection_report(
diff --git a/src/sandboxer/extensions/base.py b/src/sandboxer/extensions/base.py
index cab28c5..39ba1e5 100644
--- a/src/sandboxer/extensions/base.py
+++ b/src/sandboxer/extensions/base.py
@@ -6,7 +6,7 @@ import uuid
 from abc import ABC, abstractmethod
 from typing import Any
 
-from sandboxer.models import Profile
+from sandboxer.models import MeterQuote, Profile
 
 
 class SandboxExtension(ABC):
@@ -31,4 +31,18 @@ class SandboxExtension(ABC):
 
     @abstractmethod
     def teardown(self, handle: dict[str, str]) -> dict[str, str]:
-        """Release sandbox resources. Returns cleanup report fields."""
\ No newline at end of file
+        """Release sandbox resources. Returns cleanup report fields."""
+
+    def estimate_cost(
+        self,
+        profile: Profile,
+        inputs: dict[str, str],
+        *,
+        duration_s: int = 3600,
+    ) -> MeterQuote | None:
+        """Optional pre-create cost quote (metered SaaS extensions)."""
+        return None
+
+    def meter_actual(self, handle: dict[str, str], *, duration_s: float) -> float | None:
+        """Optional post-destroy actual cost in USD."""
+        return None
\ No newline at end of file
diff --git a/src/sandboxer/extensions/saas_stub.py b/src/sandboxer/extensions/saas_stub.py
new file mode 100644
index 0000000..6bfd959
--- /dev/null
+++ b/src/sandboxer/extensions/saas_stub.py
@@ -0,0 +1,66 @@
+"""ext.saas-stub — metered SaaS extension stub for routing and payments v0.
+
+No external provider API. Exercises estimate_cost, credits debit, and routing
+fallback without E2B/Modal credentials.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from sandboxer.extensions.base import SandboxExtension
+from sandboxer.models import MeterQuote, Profile
+
+
+class SaaSStubExtension(SandboxExtension):
+    """Simulated metered SaaS sandbox backend."""
+
+    def __init__(self, config: dict[str, Any] | None = None) -> None:
+        super().__init__(config)
+        self.rate_usd_per_hour: float = float(self.config.get("rate_usd_per_hour", 0.12))
+        self.session_fee_usd: float = float(self.config.get("session_fee_usd", 0.01))
+        self.provider: str = self.config.get("provider", "saas-stub")
+
+    def estimate_cost(
+        self,
+        profile: Profile,
+        inputs: dict[str, str],
+        *,
+        duration_s: int = 3600,
+    ) -> MeterQuote:
+        hours = max(duration_s / 3600.0, 1 / 3600)
+        estimated = round(self.session_fee_usd + hours * self.rate_usd_per_hour, 4)
+        return MeterQuote(
+            extension_id="ext.saas-stub",
+            estimated_usd=estimated,
+            unit="per_hour",
+            duration_s=duration_s,
+        )
+
+    def meter_actual(self, handle: dict[str, str], *, duration_s: float) -> float:
+        hours = max(duration_s / 3600.0, 1 / 3600)
+        return round(self.session_fee_usd + hours * self.rate_usd_per_hour, 4)
+
+    def provision(
+        self, profile: Profile, inputs: dict[str, str], host: str
+    ) -> dict[str, str]:
+        sandbox_id = self.new_sandbox_id(inputs)
+        endpoint = f"https://stub.sandboxer.local/{sandbox_id}"
+        return {
+            "sandbox_id": sandbox_id,
+            "host": self.provider,
+            "endpoint": endpoint,
+            "provider": self.provider,
+        }
+
+    def wait_ready(self, handle: dict[str, str]) -> dict[str, str]:
+        return {
+            "endpoint": handle["endpoint"],
+            "host": handle.get("host"),
+        }
+
+    def teardown(self, handle: dict[str, str]) -> dict[str, str]:
+        return {
+            "provider_removed": "true",
+            "sandbox_id": handle.get("sandbox_id", ""),
+        }
\ No newline at end of file
diff --git a/src/sandboxer/models.py b/src/sandboxer/models.py
index bcdf2b4..20f75d8 100644
--- a/src/sandboxer/models.py
+++ b/src/sandboxer/models.py
@@ -86,10 +86,32 @@ class ProfileMetadata(BaseModel):
     observability: Literal["none", "canary"] = "none"
 
 
+class RouteSpec(BaseModel):
+    strategy: RouteStrategy = RouteStrategy.EXPLICIT
+    extensions: list[str] = Field(default_factory=list)
+    max_cost_per_hour_usd: float | None = None
+
+
+class MeterQuote(BaseModel):
+    extension_id: str
+    estimated_usd: float
+    unit: Literal["per_hour", "per_session"] = "per_hour"
+    duration_s: int = 3600
+
+
+class MeterRecord(BaseModel):
+    pricing_model: Literal["self-hosted", "metered"] = "self-hosted"
+    estimate_usd: float | None = None
+    actual_usd: float | None = None
+    duration_s: float | None = None
+    currency: str = "USD"
+
+
 class Profile(BaseModel):
     id: str
     version: str
     extension: str
+    route: RouteSpec | None = None
     isolation: IsolationSpec = Field(default_factory=IsolationSpec)
     network: NetworkSpec = Field(default_factory=NetworkSpec)
     workspace: WorkspaceSpec = Field(default_factory=WorkspaceSpec)
@@ -130,6 +152,7 @@ class Reachability(BaseModel):
     remote_dir: str | None = None
     compose_project: str | None = None
     host: str | None = None
+    endpoint: str | None = None
 
 
 class SandboxStatus(BaseModel):
@@ -142,6 +165,7 @@ class SandboxStatus(BaseModel):
     reachability: Reachability | None = None
     inputs: dict[str, str] = Field(default_factory=dict)
     error: str | None = None
+    meter: MeterRecord | None = None
     telemetry: dict | None = None  # IntrospectionReport JSON when canary
     created_at: datetime
     updated_at: datetime
diff --git a/src/sandboxer/payments/__init__.py b/src/sandboxer/payments/__init__.py
new file mode 100644
index 0000000..18eeb24
--- /dev/null
+++ b/src/sandboxer/payments/__init__.py
@@ -0,0 +1,6 @@
+"""Payments and metering for SaaS sandbox extensions."""
+
+from sandboxer.payments.credits import CreditsStore
+from sandboxer.payments.metering import estimate_cost, settle_usage
+
+__all__ = ["CreditsStore", "estimate_cost", "settle_usage"]
\ No newline at end of file
diff --git a/src/sandboxer/payments/credits.py b/src/sandboxer/payments/credits.py
new file mode 100644
index 0000000..2724a94
--- /dev/null
+++ b/src/sandboxer/payments/credits.py
@@ -0,0 +1,48 @@
+"""Org/workspace credits for metered sandbox consumption."""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+
+def _default_credits_path() -> Path:
+    base = Path(os.environ.get("XDG_DATA_HOME", Path.home() / ".local" / "share"))
+    return base / "sandboxer" / "credits.json"
+
+
+class CreditsStore:
+    def __init__(self, path: Path | None = None) -> None:
+        self.path = path or _default_credits_path()
+        self.path.parent.mkdir(parents=True, exist_ok=True)
+
+    def _read(self) -> dict:
+        if not self.path.exists():
+            default = float(os.environ.get("SANDBOXER_DEFAULT_CREDITS", "10.0"))
+            return {"balance_usd": default, "currency": "USD"}
+        return json.loads(self.path.read_text())
+
+    def _write(self, data: dict) -> None:
+        self.path.write_text(json.dumps(data, indent=2))
+
+    def balance(self) -> float:
+        return float(self._read().get("balance_usd", 0.0))
+
+    def can_afford(self, amount_usd: float) -> bool:
+        return self.balance() >= amount_usd
+
+    def add(self, amount_usd: float) -> float:
+        data = self._read()
+        data["balance_usd"] = round(float(data.get("balance_usd", 0.0)) + amount_usd, 4)
+        self._write(data)
+        return data["balance_usd"]
+
+    def debit(self, amount_usd: float) -> float:
+        data = self._read()
+        new_balance = round(float(data.get("balance_usd", 0.0)) - amount_usd, 4)
+        if new_balance < 0:
+            raise ValueError(f"Insufficient credits: need {amount_usd:.4f} USD")
+        data["balance_usd"] = new_balance
+        self._write(data)
+        return new_balance
\ No newline at end of file
diff --git a/src/sandboxer/payments/metering.py b/src/sandboxer/payments/metering.py
new file mode 100644
index 0000000..159fe65
--- /dev/null
+++ b/src/sandboxer/payments/metering.py
@@ -0,0 +1,66 @@
+"""Cost estimation and usage settlement for metered extensions."""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sandboxer.extensions.registry import resolve_backend
+from sandboxer.models import Extension, MeterQuote, MeterRecord, Profile, SandboxStatus
+
+
+def _duration_seconds(ready_at: datetime | None, destroyed_at: datetime) -> float:
+    if not ready_at:
+        return 0.0
+    return max(0.0, (destroyed_at - ready_at).total_seconds())
+
+
+def estimate_cost(
+    extension: Extension,
+    profile: Profile,
+    inputs: dict[str, str],
+    *,
+    duration_s: int = 3600,
+) -> MeterQuote | None:
+    if extension.capabilities.pricing_model != "metered":
+        return None
+    backend = resolve_backend(extension)
+    if not hasattr(backend, "estimate_cost"):
+        return None
+    quote = backend.estimate_cost(profile, inputs, duration_s=duration_s)
+    if quote is None:
+        return None
+    if isinstance(quote, MeterQuote):
+        return quote
+    if isinstance(quote, dict):
+        return MeterQuote.model_validate(quote)
+    return None
+
+
+def settle_usage(
+    status: SandboxStatus,
+    extension: Extension,
+    handle: dict[str, str],
+    *,
+    destroyed_at: datetime,
+) -> MeterRecord | None:
+    if extension.capabilities.pricing_model != "metered":
+        return MeterRecord(pricing_model="self-hosted")
+
+    duration_s = _duration_seconds(status.ready_at, destroyed_at)
+    backend = resolve_backend(extension)
+    actual_usd: float | None = None
+
+    if hasattr(backend, "meter_actual"):
+        actual_usd = backend.meter_actual(handle, duration_s=duration_s)
+
+    if actual_usd is None and status.meter and status.meter.estimate_usd is not None:
+        hours = duration_s / 3600.0
+        actual_usd = round(status.meter.estimate_usd * max(hours, 1 / 3600), 4)
+
+    estimate = status.meter.estimate_usd if status.meter else None
+    return MeterRecord(
+        pricing_model="metered",
+        estimate_usd=estimate,
+        actual_usd=actual_usd,
+        duration_s=round(duration_s, 1),
+    )
\ No newline at end of file
diff --git a/src/sandboxer/routing/__init__.py b/src/sandboxer/routing/__init__.py
new file mode 100644
index 0000000..97517c2
--- /dev/null
+++ b/src/sandboxer/routing/__init__.py
@@ -0,0 +1,5 @@
+"""Extension routing — OpenRouter-style backend selection."""
+
+from sandboxer.routing.resolver import resolve_extension
+
+__all__ = ["resolve_extension"]
\ No newline at end of file
diff --git a/src/sandboxer/routing/resolver.py b/src/sandboxer/routing/resolver.py
new file mode 100644
index 0000000..d32dd51
--- /dev/null
+++ b/src/sandboxer/routing/resolver.py
@@ -0,0 +1,97 @@
+"""Select extension backend from profile route policy."""
+
+from __future__ import annotations
+
+import os
+
+from sandboxer.extensions.registry import load_extension
+from sandboxer.models import Extension, Profile, RouteStrategy
+from sandboxer.payments.metering import estimate_cost
+from sandboxer.placement import resolve_host
+
+
+def _candidates(profile: Profile) -> list[str]:
+    if profile.route and profile.route.extensions:
+        return profile.route.extensions
+    return [profile.extension]
+
+
+def _is_metered(ext: Extension) -> bool:
+    return ext.capabilities.pricing_model == "metered"
+
+
+def _self_hosted_available(profile: Profile, ext: Extension, host_override: str | None) -> bool:
+    if _is_metered(ext):
+        return True
+    if os.environ.get("SANDBOXER_FORCE_SAAS") == "1":
+        return False
+    try:
+        resolve_host(profile, override=host_override)
+        return True
+    except ValueError:
+        return False
+
+
+def _quote_cost(
+    ext: Extension, profile: Profile, inputs: dict[str, str], duration_s: int
+) -> float | None:
+    quote = estimate_cost(ext, profile, inputs, duration_s=duration_s)
+    return quote.estimated_usd if quote else None
+
+
+def resolve_extension(
+    profile: Profile,
+    inputs: dict[str, str],
+    *,
+    host_override: str | None = None,
+    duration_s: int = 3600,
+) -> Extension:
+    """Pick extension per route strategy. Raises if no candidate qualifies."""
+    strategy = (
+        profile.route.strategy
+        if profile.route
+        else RouteStrategy.EXPLICIT
+    )
+    ids = _candidates(profile)
+    loaded = [load_extension(ext_id) for ext_id in ids]
+
+    if strategy == RouteStrategy.EXPLICIT or len(loaded) == 1:
+        chosen = load_extension(profile.extension)
+        if chosen.id not in {e.id for e in loaded}:
+            chosen = loaded[0]
+        return chosen
+
+    if strategy == RouteStrategy.PREFER_SELF_HOSTED:
+        for ext in loaded:
+            if not _is_metered(ext) and _self_hosted_available(profile, ext, host_override):
+                return ext
+        for ext in loaded:
+            if _is_metered(ext):
+                return ext
+        return loaded[0]
+
+    if strategy == RouteStrategy.LOWEST_COST:
+        best: Extension | None = None
+        best_cost: float | None = None
+        for ext in loaded:
+            if not _is_metered(ext) and _self_hosted_available(profile, ext, host_override):
+                return ext
+            cost = _quote_cost(ext, profile, inputs, duration_s)
+            if cost is None:
+                continue
+            max_hour = profile.route.max_cost_per_hour_usd if profile.route else None
+            if max_hour is not None and cost > max_hour:
+                continue
+            if best is None or cost < (best_cost or float("inf")):
+                best, best_cost = ext, cost
+        if best:
+            return best
+        return loaded[-1]
+
+    if strategy == RouteStrategy.LOWEST_LATENCY:
+        for ext in loaded:
+            if not _is_metered(ext) and _self_hosted_available(profile, ext, host_override):
+                return ext
+        return loaded[-1]
+
+    return load_extension(profile.extension)
\ No newline at end of file
diff --git a/tests/test_extension_registry.py b/tests/test_extension_registry.py
index 7117709..7dd78c5 100644
--- a/tests/test_extension_registry.py
+++ b/tests/test_extension_registry.py
@@ -12,4 +12,5 @@ def test_load_vm_packer_extension() -> None:
 def test_load_all_includes_vm_packer() -> None:
     extensions = load_all_extensions()
     assert "ext.compose-ssh" in extensions
-    assert "ext.vm-packer" in extensions
\ No newline at end of file
+    assert "ext.vm-packer" in extensions
+    assert "ext.saas-stub" in extensions
\ No newline at end of file
diff --git a/tests/test_manager.py b/tests/test_manager.py
index adac6f8..43551b1 100644
--- a/tests/test_manager.py
+++ b/tests/test_manager.py
@@ -53,8 +53,9 @@ def test_create_and_destroy(store: SandboxStore) -> None:
         consumer=Consumer(actor=ActorType.ADM, project="sand-boxer"),
     )
 
+    fake = FakeBackend()
     with (
-        patch("sandboxer.core.manager.resolve_backend", return_value=FakeBackend()),
+        patch("sandboxer.core.manager.resolve_backend", return_value=fake),
         patch("sandboxer.core.manager.emit_lifecycle_event", return_value=None),
         patch("sandboxer.core.manager.resolve_host", return_value="coulombcore"),
     ):
diff --git a/tests/test_payments.py b/tests/test_payments.py
new file mode 100644
index 0000000..b10d1ef
--- /dev/null
+++ b/tests/test_payments.py
@@ -0,0 +1,98 @@
+"""Payments and metering integration tests."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import patch
+
+from sandboxer.core.manager import SandboxManager
+from sandboxer.lifecycle.store import SandboxStore
+from sandboxer.models import ActorType, Consumer, SandboxCreateRequest, SandboxState
+from sandboxer.payments.credits import CreditsStore
+
+
+class SaaSBackend:
+    def provision(self, profile, inputs, host):
+        return {"sandbox_id": "saas1234", "host": "saas-stub", "endpoint": "https://x"}
+
+    def wait_ready(self, handle):
+        return {"endpoint": handle["endpoint"], "host": handle["host"]}
+
+    def teardown(self, handle):
+        return {"provider_removed": "true"}
+
+    def estimate_cost(self, profile, inputs, *, duration_s=3600):
+        from sandboxer.models import MeterQuote
+
+        return MeterQuote(
+            extension_id="ext.saas-stub",
+            estimated_usd=0.5,
+            duration_s=duration_s,
+        )
+
+    def meter_actual(self, handle, *, duration_s: float) -> float:
+        return 0.25
+
+
+def _set_balance(credits: CreditsStore, amount: float) -> None:
+    credits._write({"balance_usd": amount, "currency": "USD"})
+
+
+def test_saas_create_destroy_debits_credits(tmp_path: Path) -> None:
+    store = SandboxStore(path=tmp_path / "sandboxes.json")
+    credits = CreditsStore(path=tmp_path / "credits.json")
+    _set_balance(credits, 5.0)
+    manager = SandboxManager(store=store, credits=credits)
+
+    request = SandboxCreateRequest(
+        profile="profile.saas-stub",
+        inputs={},
+        consumer=Consumer(actor=ActorType.ADM, project="test"),
+    )
+
+    backend = SaaSBackend()
+    with (
+        patch("sandboxer.core.manager.resolve_extension") as resolve_ext,
+        patch("sandboxer.core.manager.resolve_backend", return_value=backend),
+        patch("sandboxer.payments.metering.resolve_backend", return_value=backend),
+        patch("sandboxer.core.manager.emit_lifecycle_event", return_value=None),
+    ):
+        from sandboxer.extensions.registry import load_extension
+
+        resolve_ext.return_value = load_extension("ext.saas-stub")
+        status = manager.create(request)
+        assert status.state == SandboxState.READY
+        assert status.meter and status.meter.estimate_usd == 0.5
+
+        destroyed = manager.destroy(status.sandbox_id)
+        assert destroyed.state == SandboxState.DESTROYED
+        assert destroyed.meter and destroyed.meter.actual_usd == 0.25
+
+    assert credits.balance() == 4.75
+
+
+def test_insufficient_credits_blocks_create(tmp_path: Path) -> None:
+    store = SandboxStore(path=tmp_path / "sandboxes.json")
+    credits = CreditsStore(path=tmp_path / "credits.json")
+    _set_balance(credits, 0.01)
+    manager = SandboxManager(store=store, credits=credits)
+
+    request = SandboxCreateRequest(
+        profile="profile.saas-stub",
+        inputs={},
+        consumer=Consumer(actor=ActorType.ADM, project="test"),
+    )
+
+    with (
+        patch("sandboxer.core.manager.resolve_extension") as resolve_ext,
+        patch("sandboxer.core.manager.emit_lifecycle_event", return_value=None),
+    ):
+        from sandboxer.extensions.registry import load_extension
+
+        resolve_ext.return_value = load_extension("ext.saas-stub")
+        with patch("sandboxer.core.manager.resolve_backend", return_value=SaaSBackend()):
+            try:
+                manager.create(request)
+                raise AssertionError("expected RuntimeError")
+            except RuntimeError as exc:
+                assert "Insufficient credits" in str(exc)
\ No newline at end of file
diff --git a/tests/test_routing.py b/tests/test_routing.py
new file mode 100644
index 0000000..6ac2d67
--- /dev/null
+++ b/tests/test_routing.py
@@ -0,0 +1,59 @@
+"""Extension routing tests."""
+
+
+import pytest
+
+from sandboxer.extensions.registry import load_extension
+from sandboxer.models import Profile, RouteSpec, RouteStrategy
+from sandboxer.routing.resolver import resolve_extension
+
+
+def _burst_profile() -> Profile:
+    return Profile.model_validate(
+        {
+            "id": "profile.burst-sandbox",
+            "version": "1.0.0",
+            "extension": "ext.compose-ssh",
+            "route": {
+                "strategy": "prefer-self-hosted",
+                "extensions": ["ext.compose-ssh", "ext.saas-stub"],
+            },
+        }
+    )
+
+
+def test_explicit_uses_profile_extension() -> None:
+    profile = Profile.model_validate(
+        {
+            "id": "profile.compose-e2e",
+            "version": "1.0.0",
+            "extension": "ext.compose-ssh",
+        }
+    )
+    ext = resolve_extension(profile, {}, host_override="coulombcore")
+    assert ext.id == "ext.compose-ssh"
+
+
+def test_prefer_self_hosted_when_host_set(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("SANDBOXER_HOST", "coulombcore")
+    ext = resolve_extension(_burst_profile(), {"repo": "/tmp/x"}, host_override=None)
+    assert ext.id == "ext.compose-ssh"
+
+
+def test_prefer_self_hosted_falls_back_to_saas(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("SANDBOXER_HOST", raising=False)
+    monkeypatch.setenv("SANDBOXER_FORCE_SAAS", "1")
+    ext = resolve_extension(_burst_profile(), {}, host_override=None)
+    assert ext.id == "ext.saas-stub"
+    assert ext.capabilities.pricing_model == "metered"
+
+
+def test_lowest_cost_picks_metered_when_forced(monkeypatch: pytest.MonkeyPatch) -> None:
+    profile = _burst_profile()
+    profile.route = RouteSpec(
+        strategy=RouteStrategy.LOWEST_COST,
+        extensions=["ext.compose-ssh", "ext.saas-stub"],
+    )
+    monkeypatch.setenv("SANDBOXER_FORCE_SAAS", "1")
+    ext = resolve_extension(profile, {}, host_override=None)
+    assert load_extension(ext.id).capabilities.pricing_model == "metered"
\ No newline at end of file
diff --git a/tests/test_saas_stub.py b/tests/test_saas_stub.py
new file mode 100644
index 0000000..cba6805
--- /dev/null
+++ b/tests/test_saas_stub.py
@@ -0,0 +1,30 @@
+"""SaaS stub extension tests."""
+
+from sandboxer.extensions.saas_stub import SaaSStubExtension
+from sandboxer.models import Profile
+
+
+def _profile() -> Profile:
+    return Profile.model_validate(
+        {"id": "profile.saas-stub", "version": "1.0.0", "extension": "ext.saas-stub"}
+    )
+
+
+def test_estimate_and_meter() -> None:
+    ext = SaaSStubExtension({"rate_usd_per_hour": 1.0, "session_fee_usd": 0.05})
+    quote = ext.estimate_cost(_profile(), {}, duration_s=3600)
+    assert quote is not None
+    assert quote.estimated_usd == 1.05
+
+    handle = ext.provision(_profile(), {}, "saas")
+    actual = ext.meter_actual(handle, duration_s=1800)
+    assert actual == 0.55
+
+
+def test_provision_wait_teardown() -> None:
+    ext = SaaSStubExtension()
+    handle = ext.provision(_profile(), {}, "saas-stub")
+    reach = ext.wait_ready(handle)
+    assert reach["endpoint"].startswith("https://stub.sandboxer.local/")
+    report = ext.teardown(handle)
+    assert report["provider_removed"] == "true"
\ No newline at end of file
diff --git a/workplans/SAND-WP-0006-saas-extensions-and-payments.md b/workplans/SAND-WP-0006-saas-extensions-and-payments.md
new file mode 100644
index 0000000..6dc7c59
--- /dev/null
+++ b/workplans/SAND-WP-0006-saas-extensions-and-payments.md
@@ -0,0 +1,86 @@
+---
+id: SAND-WP-0006
+type: workplan
+title: "SaaS extensions and payments layer"
+domain: infotech
+repo: sand-boxer
+status: finished
+owner: codex
+topic_slug: custodian
+created: "2026-06-23"
+updated: "2026-06-23"
+---
+
+# SaaS extensions and payments layer
+
+Deliver INTENT pillar 4 (payments/metering) and routing engine v0 for
+self-hosted vs SaaS backend selection.
+
+**Predecessor:** SAND-WP-0005 (extension SDK — finished)  
+**Follow-on:** SAND-WP-0007 (snapshots), real E2B/Modal adapters
+
+## Payments and credits
+
+```task
+id: SAND-WP-0006-T01
+status: done
+priority: high
+```
+
+`CreditsStore`, `estimate_cost` / `settle_usage`, pre-create balance check,
+post-destroy debit, `SandboxStatus.meter` field. Docs: `docs/payments.md`.
+CLI: `sandboxer credits show|add`.
+
+## Routing engine
+
+```task
+id: SAND-WP-0006-T02
+status: done
+priority: high
+```
+
+`RouteSpec` on profiles, `resolve_extension` with strategies
+(`prefer-self-hosted`, `lowest-cost`, `lowest-latency`, `explicit`).
+Docs: `docs/routing.md`.
+
+## ext.saas-stub
+
+```task
+id: SAND-WP-0006-T03
+status: done
+priority: high
+```
+
+Metered SaaS stub extension — `estimate_cost`, `meter_actual`, no external API.
+Profiles: `profile.saas-stub`, `profile.burst-sandbox`.
+
+## Manager integration
+
+```task
+id: SAND-WP-0006-T04
+status: done
+priority: high
+```
+
+`SandboxManager` uses routing resolver; destroy loads `status.extension_id`;
+meter settlement on destroy; meter events to State Hub.
+
+## Tests
+
+```task
+id: SAND-WP-0006-T05
+status: done
+priority: high
+```
+
+`test_routing.py`, `test_payments.py`, `test_saas_stub.py`.
+
+## Deferred
+
+```task
+id: SAND-WP-0006-T06
+status: wait
+priority: low
+```
+
+Real `ext.e2b` / `ext.modal` adapters, BYOK via OpenBao, fin-hub export.
\ No newline at end of file