A friend and I were chatting over coffee awhile back, she sells here too, and the subject of incomplete orders came up and she told me when this happens to her, she asks the buyer to send photos of what she received and the packaging, and tells them she will check the warehouse closed circuit TV footage and see if she can determine whether a packing error occurred.  If no error is evident, she will forward them a stat dec which they will need to sign so that she can lodge both a claim with Auspost and a police report for mail theft.  She adds that they may receive a call from the police if they require a formal statement from her.  

She has even designed an official looking Missing Item Procedure form outlining the above, complete with an OFFICE USE ONLY BOX with Date Processed and check boxes for CCTV Checked and Stat Dec enclosed (very realistic looking) and she actually posts this along with a Stat Dec form to the customer.  She said she has never had one returned and never hears any more about it.  There is usually a lengthy gap before the buyer returns to order again, but most do return.