Proof of postage is the receipt showing you did send it. Ask for proof of postage, keep the proof of postage.
It is free at a counter and worth having, since without it a lost item cannot be claimed for. Proof of postage shows only that you sent something, not that it arrived.
1nounproof of postageThe receipt showing you did send it.
“Without proof of postage there was nothing to claim on.”
proof of postage is a receipt.
Practised in Bank & Post Office