Sorry to correct you recped, here is the definition from PayPal:
What is proof of shipment?
Proof of shipment is documentation or online verification from the shipping company that shows the shipping date, the address to which the package was sent, and official acceptance from the shipping company (for example, a postmark). It differs from proof of delivery because it doesn’t have to show that the package was actually delivered, just that it was shipped. The proof of shipment doesn’t need to show the full street address since some shipping companies don’t include this information. The city/state or ZIP code (or equivalent) are normally acceptable.
So, my copy of the postal receipt with stamp from the post office qualified as PROOF OF SHIPMENT.