It's against ebay rules to charge a buyer more postage that is stated on your listing.
If you've printed an incorrect postage label you can cancel it and get a refund. On your orders page select Postage Labels on the left-hand side. Go to the dropdown actions menu next to the label and select Void Label.
You can buy your own postage label at your post office. Remember to add the tracking number and mark as dispatched manually via the dropdown menu for the item on your sold list.
