You have to cancel/refund the whole 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.
Yep, it's long winded no denying that, and for the unused label this is how to receive a refund, it depends who originally paid for the label.
If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days
If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling. Also ensure you add the replacement tracking number of the service used, and it shows delivery.
