usually its the top one on the list in amazon. You can also choose to make one the default and then they will use it until you change the default. If you do the long checkout through your cart instead of one click ordering, then you can choose which card is used for each order.
Thanks for the answers, the problem is it's not the top one on the list that I want to use but I can't see how you make a certain card default it does not seem to give me that option.