Hi, the response I've received from support was:
"I have reviewed your account in detail and see that based on your physical location you will not be able to make any payments with Standing Stone Games at this time. I apologize but I cannot assist you with this issue nor can I provide any work-around."
It's unclear just how they determine that, but apparently it's some sort of account flag. Typical workarounds like VPN or even remote access did not work. This means that you likely won't be able to buy anything even if you were to, say, hop on a plane to the U.S. My guess is it's some sort of an account ban, but applicable only to the store.
As for the in-game store, if you have that issue - just forget it. I've had this issue for years, since the introduction of the new DDO Store, and I wouldn't count on getting any help from SSG / support. It's not getting solved / fixed. May or may not be related to the issues with the new store.
DDO Market is the only thing that works, for now.
If you discover a workaround, please do let me know via a Private Message. Thanks!