Cancelling an order without buyers agreement will get a defect on your selling account.
Ebay seems incapable of accepting that until you have a sale you will not know where the buyer lives. Unlike Royal Mail Couriers do not have ONE PRICE for the whole of the UK, but buyer can only pay the amount shown on your listing and you cannot ask for more.
If you are having to use Simple Postage use Royal Mail to a avoid this situation, as many parts of the UK can prove very expensive if using couriers.