You can now get a GST receipt from Aliexpress.
On the order details page next to the "Confirm Goods Received" button you'll find a "Download Invoice" button, click it, then in the dialog that opens click the "Receipt" link, no need to fill out the form or submit I don't know what that's about, just click the link and you'll get a suitable PDF GST Receipt (well, looks good enough, has a GST number on it).