After operator approval, SKUU asks the fulfilling seller to create one return for one seller order. SKUU resolves and splits the approved consumer request internally before sending this minimal request. external_ref is the idempotency key: exact retries return the same HTTP 200 receipt, while a conflicting reuse returns 409. SKUU retries transport errors, 429 and 5xx responses using the identical signed body. Enable only after bilateral conformance in TESTING and explicit activation.