If you are having trouble getting your payment through Amazon payment system to process, it's possible they are defaulting to an old credit card. Here is a screenshot which details how to change which card is used for processing your payment. 

If after changing your credit card, it still won't go through, it's possible your device is logged into someone else's amazon account. If you look on the upper right in the screenshot below, it says "hello, [name]". Make sure it shows your name, or you might be trying to use your friend's amazon account, which will prevent it from going through.

If  you still have trouble after changing the credit card and double checking the account, send us an email and we'll figure out a way to get your payment to go through.