The horror stories I know were more with FedEx than with USPS.
The most outrageous example was when a friend had sold a guitar amp, and shipped it double boxed. The inner box was the original box from the manufacturer. When it got to the person he sent it to, there was a huge hole right through the boxes and the speaker itself. Someone at FedEx had obviously driven a forklift right through the amp! FedEx actually had the gall to suggest that they were not responsible because the item had not been packaged properly! I guess you are supposed to use a titanium container? He raised a big stink and they eventually paid up, but not without a lot of foot dragging and hassles.
My friend's incident occurred with UPS, not FedEx. Also, it turns out that they were sticking to their guns not to pay, even though a forklift had been run clear through the box. The reason he finally collected was not that the item was damaged. He had the guy who bought ship it back, and it got lost in the return trip. So, he got paid not because of their negligence in destroying the amp, but because of their negligence in losing it.
Amazing.
Do you ever wonder how these people with this "insurance adjuster" mentality live with themselves after finding every imaginable way of wriggling out of their responsibilities?