I think you need to assess your post policies overall and do a rethink on the basis you have them as your policies.

For example, you indicate in your terms that you do not post to post office boxes, yet you send via Aust Post and a PO is one of the safest delivery destinations IMO.

I would also most certainly be on the phone to Australia Post to discuss the issue of that volume of your outgoing mail going missing.

I would encourage you to have look at the recent threads on the Powersellers Board that has some useful seller post tips (thread title not the most obvious but includes word guys).