See, these "proof of payments" are fucked. Nowadays people are not even using signed messages anymore, but blockchain transactions which could be theirs or could belong to some random person. Thus a person could commit fraud by producing an arbitrary transaction on a block explorer as a "proof of payment".

This "proof of payment" is only valid between buyer and seller, and meaningless to everyone else. There is no way (currently) to prove that you actually sent Bitcoins somewhere.