Do you make a claim through the post office for every item that disappears? Standard non insured mail is covered for $50 value. It's a bit of stuffing about but if you aren't going to add signature for everything you should claim every lost item. If it's $29 value, claim the $20 plus the postage as the total so you aren't out of pocket. 

Maybe if more people did that, genuinely missing items wouldn't be as common. Also it means they will investigate meaning if it is dodgy they may discover this.