There is already support for a 'primary' address (I call it the default address). There is no guarantee that every user will have one (since the module can be added to an existing system), but every user with at least one address will have a default address.

Of course, anonymous users have no associated addresses.

For an international store, you would probably want to give the user a choice of currency to use, but I suppose the default address, when available, might be a starting point.

Tony Freixas
tony@tigerheron.com
http://www.tigerheron.com