Possible Clues:

• Proof of purchase, bit held up by those starting rail trip
• Confirmation of right to hold gun overturned
• Record of expenditure
• Lengthy register printout from CVS
• Right part sent back -- time to find proof of purchase
• Acknowledgement of item reviewed in Radio Times at first
• Proof of payment made
• Acknowledgement of order, not the first I received
• It often comes with a 'Thank You'
• Price set wrongly with no end of fuss? It comes from the till