You could 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.
Or after you've bought the postage generate the original prepaid label to mark the item as dispatched. Then add the new tracking via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That needs to be done on a desktop browser. Buyer will receive refund for the unused label in 30 days.