Of course ebay won't let him pay, you would need to quote TRACKED POSTAGE cost to Bulgaria,that is your responsibility. If he was allowed to pay £15 Postage, would you be happy to pay the real postage and post, and is the buyer aware of how much IMPORT DUTY he would still have to pay.
As already said as you obviously have no idea how the system works, you risk losing a lot of money and should cancel the sale.
