You cannot combine them afaik. I used them in Amazon US. It's a like a regular CC/DC. Can you combine two to make a single purchase? Nope, same way you cannot do it with a virtual one either. Gift card purchase should be possible. Try different variants, e-gift cards/email gift cards/send gift cards in various method, something's gotta work. Add money to amazon pay will not work because it auto detects as foreign cc even though it might be not because afaik VISA gift cards are still not officially available here, I might be wrong though.