all parcels sent via australia post now have a tracking number included, the number is on the printed sticker they put on the parcel if it is not in a parcel post bag.

Stress to your buyers that if they don't choose registered post or insurance for items over a particular $$ value there is no responsibilit on your part if the item goes missing or arrives damaged. 

If a customer says they haven't received an item and there isn't a tracking number, ring Australia Post and ask them to check with the closest post office of the buyer, they will hesitate to do this, but be polite and persistant. 

I don't have many items that go missing, but I never refund until I can confirm whether the item is in fact missing. I have been scammed one too many times.