Please clarify your set up. A typical Packing Slip often has a copy within the shipment. A shipment may be the whole or part of an Order. When I get a shipment related to an Order, the Packing slip is used to ensure all parts are properly received.
Just checked wikipedia and found
Code:
A shipping list, packing list, waybill, packing slip (also known as a bill of parcel,
unpacking note, packaging slip, (delivery) docket, delivery list, manifest or customer receipt),
is a shipping document that accompanies delivery packages, usually inside an attached
shipping pouch or inside the package itself.