Yup, it's Amex. The Marketplace doesn't like it.I used MC to purchase something that was not USD and it works. I've been hearing that Visa is another problem with non USD apps.
Not too sure about Amex but I'm guessing that yes, that is the problem.
Go into your Google checkout acct and swap out the card w/ MC and that will definitely solve the problem.
Yeah I've found out that you can't purchase an item for non-us dollars with Discover either.