This is the trouble with using ebay and Couriers to deliver.
Buyer just pays to you the sale price plus the postage you put in the listing. From that you have to buy the postage.
You cannot ask buyer to pay more, but unlike Royal Mail, couriers do not have one price for all parts of the Country, so if your buyer is in remoter parts such as Scottish Islands then the quoted cost will be inadequate. They also do not in most cases cover damage or pay more than £20 compensation if lost, but you have to refund buyer in full.