‘Outlook.com account is growing too fast’ – How do I purchase a larger Outlook.com account in Australia?

I received an email from Outlook.com that my 'Outlook.com account is growing too fast'.

I've had this account for many years (albeit with Windows Live before the move to Outlook.com obviously) and never had a problem before like this.

Therefore I'm looking for solutions and as I understand it a paid account will give me additional space.

However when I try to purchase, it requires a US billing address:

http://windows.microsoft.com/en-au/outlook/ad-free-outlook

Is there an Australian order form for this service?

Thanks,

Tim