Proof of postage is not proof of delivery. I an item is late by one day then the buyer can open a Not received dispute. As you say it could be because of Christmas.
if they open a dispute then you have to refund. if you dont and the buyer escalates to ebay ebay would force the refund
Problem is if you refund and then the item arrives you are relying on the buyers honesty to repay.
Some will and some don.'tDepending on how long it has been you may be able to claim from the courier or RM for late delivery but I would not hold my breath.
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