My local Post Office printed a delivery label and then measured the parcel and rejected it because it was slightly to large. They've taken payment but offered no service. Can I ask the buyer to send funds to cover a new postage label?
For the parcel it's best to cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.