If it were an American vendor, I would say tough luck to them. But since they're Egyptian, and I kinda feel bad about the wages they make anyway, if I were in that situation I'd probably just pay the difference
I do agree that it's proper business etiquette that they eat the cost since you agreed to the original quote, but I'd say if you are satisfied with their service, it might be a good gesture to pay it just to keep peaceful relations if you plan to order from them again. I agree that Western Union is rather pricey, so maybe they could attach the $20 on to your next purchase?