Hello,
If the overseas company earns more than $75,000 AUD p/a, they are required to collect GST. The GST applies to cost of item and cost of postage from memory.
Custom imports taxes should not include the 5% tariff fee, as per listed as item/s 9005 (Chapter 90) of the tariff classifications.
Steve
GST
https://www.ato.gov.au/business/inte...for-consumers/
Tarrifs
https://www.abf.gov.au/importing-exp...hapter-90#tele]
Ps. 9005-xx items are listed as "BINOCULARS, MONOCULARS, OTHER OPTICAL TELESCOPES, AND MOUNTINGS THEREFOR; OTHER ASTRONOMICAL INSTRUMENTS AND MOUNTINGS THEREFOR, BUT NOT INCLUDING INSTRUMENTS FOR RADIO-ASTRONOMY:"