Probably too much defending of shipping companies in this post, but I've got enough experience in that field that I figured I'd share my thoughts.
Enigma and Rob appear to have this pretty well figured out. I worked for UPS for seven years both as a package handler in the sorting (warehouse) facility and later in management during and for several years after college. While I can't write definitively regarding the USPS, I doubt they operate all that much differently in their package sorting and transport facilities than either of their private competitors mentioned in this thread. There are rules about package handling and they are generally obeyed by the employees, who I would imagine would like to keep their jobs. But you are talking about a very large volume of packages making their way through these facilities out of trucks/bins and onto conveyor belts and down chutes and stacked into trucks or airplanes to later have the process performed in reverse. In cases of non-air delivery across several states or across the country this process can be repeated two or three times. There are many opportunities for things to go wrong and for packages to be damaged, destroyed or even to disappear entirely. That most of us have had few problems with damage to the live plants we ship despite the process they endure in transit (and the above doesn't really come close to describing what it's like to watch the controlled chaos that is an active package processing facility during peak volume times - the logistics are daunting) is a testament to the hard work done attempting to preserve the integrity of your packages by the vast majority of employees in these organizations.
Suggesting that no packages should ever be damaged or their contents destroyed or that companies/govt organizations that do should close up the shop and make way for companies that can do it better is fine, but ignores the fact that such is already the case as far as competition. UPS and FedEx as well as other, smaller package transport companies are privately held, yet they still damage items shipped with them just like the USPS. Perhaps they have lower damage percentages, but I doubt those numbers are out there for comparison. The USPS is also much less expensive for most shipping options. There will be the occasional damaged package as long as items are shipped in boxes that can be damaged and are packaged in a way that allows their contents to be tumbled around inside...or until we all have private transporters ala Star Trek. I wouldn't hold my breath on either development.