Since it's after 01/07/2018, I thought I'd post an update.
As we already figured would happen,
Shipito is now complying with the Australian
Treasury Laws Amendment (GST Low Value goods) Act 2017.
GST applies to the
value of the item (presumably this will be derived from the customs deceleration the recipient fills out) as well as the shipping and insurance costs.
I successfully ordered an item from Amazon (US) dispite being in Australia. It doesn't look like Amazon (at this stage) is forcing users to their .com.au domain, but if you select an Australian shipping address, it tells you the item is not available for shipping to that destination.
If you provide Shipito with an ABN, you are not required to pay GST. I guess at the end of the day, what's stopping anyone from just quoting any ABN of any random business on their Shipito profile to avoid paying tax?