You can't add to the existing postage. You either have to cancel the current transaction with buyer's agreement and relist with correct postage for them to buy again. Or buy the correct postage yourself and change the tracking on the existing transaction. The buyer will receive a refund for the original unused label.
In theory you could just stick stamps on the parcel but there'd be no tracking so no proof of delivery. Not necessarily a problem if the item is not expensive. Open the original label to mark the item as dispatched. We can't do that manually any more.