Idempotency
Retry the request, not the money.
Every financial POST requires Idempotency-Key. Repeating the same request with the same key returns its original result.
Same key + same request
Returns the original HTTP status and response. A connection drop can be retried without guessing.
Safe to retrySame key + changed request
Returns 409 idempotency_key_reused. A key can never be reused for a different request.