Sale in, purchase order out, tracking back

Splitting is the normal case in this trade, not the exception. Rates are computed per origin and shown as one figure at checkout, but the tracking is per parcel — because a shop that shows one tracking number for an order arriving on three different days has told the customer something untrue and spends the week explaining it.

A drop-shipper's suppliers are the whole of their business. The tracking a customer sees carries the carrier and the parcel and never the warehouse it left, by construction rather than by a setting — otherwise anybody who buys one item learns where to buy it cheaper.

Routing picks the lowest cost among suppliers who can fill the whole line. A supplier two pounds cheaper and three units short is a split shipment, a second parcel and a support ticket, and the arithmetic that calls them cheapest loses money. A supplier who publishes no stock feed — most of them — is treated as available, with the question asked properly on the purchase order.

Unroutable lines come back named, with the reason. An order line that vanishes between the shop and the warehouse is a customer who paid for something nobody is sending, and it is discovered by them rather than by you.