Best thing is to explain problem to buyer then cancel/refund the transaction with their 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.
The alternative would be to buy and pay for the correct postage yourself. Add that tracking number via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That needs to be done on a desktop browser.
The cost of the original label will be refunded to the buyer automatically in 30 days.