I see you've sold a few glass things - an item should be packed so well that Aus Post can use it as a football and not damage it. It's also possible that the buyer dropped the item - however you'll never be able to prove it.

Aus Post will possibly put the onus back on you to prove that you wrapped the item sufficiently. They may also request that the buyer take the packaging to their nearest PO for assessment.

Since the item was low value, it might not be worth all the hassle (especially if buyer is resistant to making a trip to the PO).

It might just be better to refund them and move on.