I am Belgian, bike is Belgian and my green card is Belgian; my point is: All the balkans are covered in my green card, except for Albania and Kosovo.
Kosovo = fixed price that is marked on the booth/office where you go into to buy it: 15€.
Albania = asked my insurer and got a separate (free!) green card for two weeks there.
So the whole thing starts with the fact what country is included in you national green card and which don't, so you're right to make a difference between US/EU/UK etc.