If this is a recent change, it's a step in the right direction. Hopefully, that message occurs when any add-on purchase is attempted, not just US add-ons. The confusion around buying add-ons would be much alleviated by simply including instructions on the purchasing page like this:
Buying an add-on is a two-step process
1 - Purchase add-on to load funds onto account from credit card.
2 - Repeat purchase process again to buy add-on with the funds loaded.*
*If sufficient funds are already available in account, then the purchase process need only be performed once.
