See if you can cancel/refund the transaction with buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist for them to buy and pay again.
If not you'd either have to buy the postage yourself. Or ask the buyer to open a "not received" case after the estimated delivery has passed, to get a refund that way. Then relist for them to buy again. If it's not too expensive buying your own postage might be the better option.
@whitt_4610