We don't know the full circumstances or how a surcharge fee has become payable. It could be as simple as the stamps having fallen off (a regular occurrence) but my guess is that you may have sent it by courier, and perhaps not realised that a surcharge was payable to the buyer's area. (Unlike Royal Mail, couriers can and do surcharge for remote areas. Is it in Scotland, where most of the disputes seem to arise?)
Anyway, the bottom line is that the buyer is entitled to have it delivered for the price you stated in the listing. Any surcharge above this is between you and the buyer. Buyers can either refuse to accept delivery, in which case they can use eBay's money back guarantee to claim a full refund, including delivery. Or you can agree to send them the extra cost to pay the surcharge.
I'm afraid the ball is in your side of the court. Whatever you decide, you won't be reimbursed if you have to pay a surcharge to complete the delivery.
