If paid by credit card, then you can dispute under the Fair Credit Billing Act if you meet the timeliness provision.
FCBA disputes provide substantial evidentiary provisions.
This was finally decided in my favor yesterday. While I could not provide "valid" documentation to prove they recived the item, PayPal finally agreed to take screen shots of my feedback page, the recent fraud negatives on the buyer's page and the USPS confirmation email that came with the shipping label and then forwarded them over to the buyer's financial institution as proof purchase. I swear it was like pulling teeth with these people.