All my listings at the moment are for anywhere in the world - when I list, I just use the drop down menu in the postage section to add the appropriate costs for europe and worldwide (and it's easy enough to get the costs off the Royal Mail website).
If there is something that I don't want to sell outside the UK then I only list UK postage.
I always put a note to say that if a winner lives outside the UK, I will only accept payment by PayPal.
However lolly - why don't you want to sell outside the UK? It's no more trouble than selling in the UK - a good third of what I sell goes to "foreign parts" and it's no more difficult than sending to a UK address.
With your Bulgarian person, all you needed to do was add the correct postage when you sent them the invoice - but if it was too late, you just bill them through PayPal for the additional amount.