Selects one request-scoped rail and returns its short-lived capture action. Saved payment-method identifiers and raw financial credentials are never accepted.

Returns 409 InvalidStateTransition with detailCode payments.PaymentMethodNotAllowed when the Invoice does not accept the requested rail — a conflict no re-read can clear, so retry with the other rail rather than a fresh ETag. BankTransfer also returns it when the payee has no wire instructions. Reusing an Idempotency-Key with a different body returns 409 IdempotencyKeyConflict with detailCode payments.IdempotencyKeyConflict. An upstream response that cannot be projected onto this contract returns 503 ServiceUnavailable with detailCode payments.UpstreamContractViolation.