a sales slip, label, box top, or other item associated with a product that is presentable as evidence of actual purchase, as for claiming a refund or rebate.
Examples from the web for proof-of-purchase
Dillard's requires a receipt or proof-of-purchase label and packaging on returns.
Contemporary definitions for proof-of-purchase
adjective
pertaining to a symbol or label on a product that can be removed by the buyer to prove purchase of a product in order to receive a rebate or refund