Thanks - I did not khow this.  Thjis was eBay's advice to me earlier today:

"All services you use on eBay like final value fee, final value fee on shipping, postage labels, listing fees and store subscription fees are included for GST. Just a heads up, we will charge you per invoice only. For example, your invoice for May is $660.14. You need to multiply $660.14 x 10%. The answer will be $66.01 and that's the tax that you will be paying. It's highly suggested to place your ABN here, so you will be excluded to the tax."  

So if they do this we will be charged GST twice for postage lables unless we record our ABN with eBay (if you have an ABN).