The credit step of POST /v1/fulfillment-orders/:id/fulfill addressed by the order: its only fulfillment order (404 when it has none, 409 multiple_fulfillment_orders when it has several: address one by its id).

Scope: fulfillment:write, channel:write or admin. A permission group made before the Channel API scopes existed grants it as fulfillment-orders:manage.

Idempotency: send Idempotency-Key; a retry with the same key replays the first answer.

Rate limit: write class, 1,200 requests per minute per tenant by default (read class 6,000). Every response carries X-RateLimit-Limit, X-RateLimit-Remaining and X-RateLimit-Reset; a refusal is 429 rate_limited with Retry-After.