I have sold an item and the simple delivery (*bleep*) does not cover the cost of postage due to packaging weight. I used to be able to book my postage once I had packed my package and my postage charge covered this.
The best thing to do is 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.
