Support & GST (Australia)

I am setting up a site for small businesses to get websites with ecommerce. I am based in Australia, and so need to charge any Australian subscribers GST (10% tax) onto of the subscription price.

I am using the Supporter plugin to give acces to Premium Themes and Plugins.

Marketpress gives the ability to add sales tax, and I was wondering if there's an easy way to add GST to Supporter signups and recurring billing when the subscriber lives in Australia?

I was thinking that I might put a drop down "country of residence" box on the sign up page, and use this as the basis for the GST addition.

Is there an easy way for me to add this functionality?

Also: How does Edublogs deal with the GST issue? I know Edublogs was founded in Melbourne, but the subscription options do not include adding GST to the cost. So is the company registered outside Australia, or does the organisation just include GST in the price, and so get an extra 10% for every non-Australian sign up?

This issue is somewhat of a pain, but something I have to address with my site (death and taxes...and all that ).

Regards