What is the difference between the waybill number and a store order ID?

The waybill number (invoice number) is the carrier-issued code Pony Express assigns to a physical shipment, and it is the only reference that returns scan events. A store order ID identifies your purchase in the retailer's system but is not recognized by the carrier's tracking. If your tracking returns nothing, check that you are using the waybill number rather than the order ID.