Proof of shipment (or sending) is different from proof of addressee.  I recall proof of addressee details it is mentioned under eligibility section for seller protection.

Whilst paypal appear to accept lodgement receipt with postcode details as proof of addressee on occassion - their agreement is written in such a way that it is not a given.

I would never send anything prepaid without filling out a AP lodgement form and having stamped by post office at lodgement and attaching tracking number to that.  It takes a few extra minutes, but well worth the peace of mind - as well as providing AP addressee proof in missing parcel investigation when pursuing compensation.