Possible Clues:

• Proof of purchase: Abbr
• Sales slip: Abbr
• Proof-of-payment doc
• Rebate requirement, at times: Abbr
• Receipt (abbr.)
• A.T.M. printout: Abbr
• Tkt. stub, e.g
• Dept. store slip
• Return request: Abbr
• Exch. need