[BUG] ConfirmShipment API ignores shipFromSupplySourceId and reverts to default address #5163
Description
There is a significant discrepancy between the behavior of the SP-API ConfirmShipment endpoint and the legacy XML Shipping Confirmation feed.
When confirming a shipment for the first time on a new order, providing a specific shipFromSupplySourceId via the SP-API is ignored. The API returns a 204 Success code, but the shipment is recorded using the seller account's default ship-from address instead of the address associated with the provided Supply Source ID.
This behavior does not occur when using the XML Feed for the same operation; the XML Feed correctly honors the supply source.
Steps to Reproduce:
Identify a new order that has not yet had any package information or fulfillment data submitted.
Call the ConfirmShipment operation.
Include packageDetail and a valid shipFromSupplySourceId that is different from the default account address.
The API returns 204 No Content.
Check the order details via getOrder or in Seller Central.
Actual Results:
The ship-from address is the account's default address. The shipFromSupplySourceId provided in the request was ignored.
Expected Results:
The ship-from address should match the location associated with the shipFromSupplySourceId provided in the ConfirmShipment call.
Additional Context:
Developer support has claimed that the ship-from address is "set when the package is first created." However, in this workflow, the ConfirmShipment call is the creation of the package. There are no prior API calls (such as POST_ORDER_FULFILLMENT_DATA) made for these orders.
Furthermore, the XML Feed performs this exact task successfully for the first package submission, proving that the fulfillment logic at the backend level supports setting the address at this stage. The issue appears to be specific to the SP-API implementation of the ConfirmShipment endpoint.
Impact:
This prevents multi-location sellers from accurately reporting fulfillment origins via the SP-API, leading to incorrect shipping calculations and compliance issues.