Commcat-I know tandem dump trucks dump their loads but have never seen a USPS trailer dump a load. Maybe I live in a different part of the country? I used to drive for a USPS contractor & everything was loaded into a cart that was wheeled on/off the trailer.
Tvad-Depending on what time of the year it is & how far away the destination determines whether the package will fly or not via Priority. There is no guarantee for 2-3 day delivery. They will *try* and deliver in 2 or 3 days.
Speaking of flying, that is the best way to ship something. Ground packages get put on the conveyor belt & get handled many more times than air packages.
Regardless, always pack your gear to survive, at minimum, a chest high drop. If you don't feel your package is secure enough to survive a chest high drop, don't ship ground or rethink your packaging.