Register a return on the selling shop's own order, for an item the selling shop sold and another shop shipped, so the selling shop can refund the customer. Each request belongs to one order; separate returns may create separate requests.

• Idempotency: external_ref. Same reference, same return; different data is 409.
• Response: HTTP 200 with external_ref and the store's return_id.
• Retries: SKUU retries a network failure, 429 and any 5xx; the schedule is on Errors and retries. Any other 4xx stops the flow and alerts SKUU customer service.