what's "unsubstantiated" is your lack of understanding... someone has to vette them, and he made a purchase, you did not,... duh
He made a claim and is unsubstantiated. He claimed that he paid for the order he placed. So in order to substantiate the claim, he needs to provide a proof of payment (a.k.a. ID transaction).
he provided all the proof you need, but thats up to him... i cant see any harm in it
if you were savvy, you could match the amounts, etc