Maybe there's no requirement for ebay to show the GST because they're not really selling you the postage. They're more or less acting a broker or middle man and you're still buying it from AP.
Did this get resolved? I know you used to be able to get a Tax Invoice for each postage label (not available immediately, had to wait until it was all processed, maybe after the item was delivered). But now with the new eBay managed payment system I can't find anywhere that shows that postage labels include GST, and Tax Invoices for Postage Labels don't seem available anymore. And the link above doesn't work anymore.
