You can update part of the return items information via API. For now, you can update merchant tags and images.
The external_id should be consistent with order.items[x].source_id if you use commerce api to feed order data into AfterShip Returns.
You can also take the external_id from received webhook payload data.return_items[x].external_id
RMA number
Item id
Merchant tags you want to add this return item. You can manage tags in AfterShip Returns Admin.