Well shopmate are auspost and auspost are paid by the au government, so it's all part of the same scam.
As such they treat all items that come into your US mailing address as a "new item being imported to avoid paying for it at aussie retail prices", even when it's something second hand.
That, and shopmates already high prices, are good enough reasons to avoid the service in my opinion.

Here's some more info for everyone.
As my accountant explained it to me, anything purchased through a digital platform such as amazon or ebay has to have all tax included in the final price you pay if shipped directly to australia.
So anything that an ebay seller sends to you can not be taxed on this end because the platform (ebay) is supposed to pay the australian government the taxes on that item due to collecting it from you in the final cost.
Expect to see a section labelled "taxes" during final checkout when purchasing on ebay after june 30th.

Now if the items are sent to a location in america/japan/etc (such as a friends place or other shipping forwarder) then forwarded to you as merchandise you will pay tax at the post office when it arrives here.
The exception to this is if the shipper has collected tax on the australian governments behalf, I know tenso (a japanese shipping forwarder) has stated that they are collecting the taxes, so you'd be paying those when you pay tenso their fees.

If however the items are sent to you as gift (either from a friend or from a shipping forwarder that isnt collecting taxes) you should not pay any taxes.
Also when sending as a gift it can help to have, for example, "used toys" in the description to further show that this is not an item that can be bought in australian retail outlets (the thing that this tax is supposedly in place to combat).
If the forwarder/retailer has charged you taxes in the final price then it doesnt matter if the customs form says merchandise or gift.

Apologies for the wall of text but hopefully this information is useful and accurate (which I assume it is coming from an accountant).