Sellers are urged to follow best practices and only mark shipments as “Shipped” once (i) shipping is purchased for the shipment; and (ii) the package has been handed off to the carrier for delivery.
Today, sellers are able to Buy Shipping (CreateShipment) for orders they have already confirmed as “Shipped” in the following ways: (i) via Seller Central; (ii) using ConfirmShipment via the Buy Shipping API; or (iii) via the Feeds API. However, when sellers try to Buy Shipping for a number of packages different to what was originally confirmed, the Buy Shipping button (CreateShipment) fails (e.g. the seller originally confirmed two packages, but attempts to Buy Shipping for three).
As of February 17, 2020, sellers are no longer allowed to Buy Shipping for orders they have already confirmed, without first cancelling the shipments with the CancelShipment option in Seller Central prior to using CreateShipment on Buy Shipping.