Hello,

About Australia, i don't know. I have already some user purchasing from Australia without tax added.
I don't manage the transaction, Share-it does it for me (and it's more time for me to work on development, and not in administrative tasks! )

I could recommend you to contact directly the Share-it Customer service, as i don't know if there's a special reason regarding to exchange rates...
Or maybe this is explained somewhere on the Terms and Conditions ?

Don't hesitate to send me an email to keep me informed of the reason

Best Regards,
Cyril