Cancel/refund the transaction with buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy again.
Or buy the postage yourself and change the tracking number on ebay via View Order Details > Postage > Track Package > dropdown arrow. That needs to be done on a desktop browser. If buyer paid for original label it will automatically be refunded to them in 30 days.
