You would not believe how much stuff they are unable to deliver each month. An entire warehouse full. Of just textbooks, 2 to 3 semi truck loads. Another 5 to 10 semi truck loads of regular books.
These included items where the packaging was damaged, or label torn off or illegible address, etc.
We would go to the auctions and purchase tons of textbooks and started noticing that in some cases we were buying back books that we had shipped to customers and had been lost.
They always made claims that they made every effort to return the lost items but as a last resort, they would end up at the auction.
They would usually cost us several thousand dollars a month in lost shipments so I had a distinct rubber stamp with bright red ink made up that said "Attention USPS: if this book is lost in shipping, please return to the following address..."
We stamped the inside cover (which is usually a blank white page) of every single book we shipped for a year. We never once had a lost book returned to us. But every month, we would buy up tons of lost books and when opening them to grade and stamp, we would frequently find our big stamp and it was always blacked out with magic marker.
So instead of having their "investigators" trying to find where to return books, they were spending time destroying evidence to cover their butts so they could continue to profit off their own incompetence.