As far as I'm aware of, Napster determines the country from your IP address you are currently using, regardless of where you originally set up your account. If this IP is not recognised as one of its locations where it is authorised to sell music, it will prevent you from placing an order.
This issue also happens with many other country-specific services. Customers on satellite based Internet connections are usually the worst affected, since as a satellite signal can cover multiple countries (especially in Europe), many country specific services will block customers using satellite based Internet.
I would suggest contacting Napster's support about your issue to see if they can unblock your IP, particularly since you have an existing account with them.