For my items - I have them tracked and insured - which I make sure is included in the shipping methods but I also add a signature (where applicable) at no extra cost to the buyer - I understand that not all PO are the same depending on where you live ...
How long as it been since the package was sent ? Having it tracked does offer proof for arrival, but as the others were saying, maybe the buyer's neighbor received it instead ? I would talk to the buyer about contacting both their neighbor and PO only after you have spoken to your PO first and try to determine what happened to the package. You could always open an investigation thru your PO to track down that package
:)