Hi ,  i realise of no immediate comfort to you , but there must be a low cost solution regarding proving Seller parcel contents integrity

With the US on-shipping service  I use there is an option to have the contents photographed and uploaded for online inspection prior to redirected dispatch, so maybe a similar service being offered by Australia Post could be viable .

Specifically to have the contents image scanned in within the unsealed parcel by counter staff at Australia Post as part of tracking record prior to the parcel being sealed 

As this would be an Aust Post procedure , the images out of control of the Seller ,  would prove item sent in the case of a claim against by Buyer or an insurance claim