Pre purchase item code wrong zone

[Cat_Girl]

So use my VPN for the us store?

Check the link that Mosha posted, you don't need a VPN.

I would checkout as a Guest, just enter your email address (required) and follow the instructions, though since your account is NA you might have used the site before and can just login.
You can use AU credit/debit cards no problem, just don't use a VPN as it causes issues.

Make sure the site doesn't auto correct to the AU or EU one (make sure .na.store is still in the url and it doesn't change to .au or .eu).