I work at UPS. I can tell you folks firsthand: avoid sending your bass, or amplifiers through delivery services if possible. Whether the box says, "Fragile... Do Not Stack... And So On," it almost always means nothing to the loader or unloader.
If it weighs over 70 pounds, there is 99% guarantee that it will be "dropped", (hopefully gently,) from ~3 or 4 feet, from the conveyor belt to what we call the bulk chute. (Anything over 70 pounds, 6 feet in length, or 4 feet in height can not go through the "standard" sorting process, and is considered bulk.)
My advise for shipping guitars and basses is to make a case out of wood, and stuff it with protective material around the actual case.
I am not saying stay away from delivery services, because something like a pedal or two, cables, small accessories or something should be fine- but when you have a long guitar case, or a heavy amplifier, I feel I must warn of the dangers involved.
That's good info from an insider, thanks.
It's almost impossible not to use UPS/Fedex etc. I know of no other means to convey items from one part of the country to another. I don't know if USPS is any better for parcel handling, but if anyone has 1st-hand experience working for USPS and doing parcels, I'd love to know.