Each product line now carries the product ID assigned by the shop it came from — the Shopify variant ID, the Amazon ASIN, the eBay item ID, and so on, depending on your connected shop.
It appears read-only next to the SKU in the Ordered items section of the order page, in display and in edit mode.
This prepares the product identifier data that carriers will require for EU customs clearance, which needs both a seller product code and a manufacturer product code.
For API users: POST /orders and POST /shipments now accept products[].item_id, returned on every order and shipment product.
