I've had good success shipping fragile things (glass and neon tubing) with both Federal Express and the USPS; I believe Deering ships their banjos with Federal Express. I don't use regular air freight; I'm fairly far from an airport and have had bad luck with it in the past. If I were shipping guitars on a regular basis, I'd want not only a hard-shell case but also a crate made of something like plywood and 2x4s to put the case in, with enough packing in the crate to insure the case can't move around. The late Gamble Rogers had a steel case for his guitar and he packed the guitar inside the case with socks and underwear. This worked for him (well enough that he even checked his guitar as baggage on the dreaded Eastern Airlines). I think the key is to insure the guitar is well-padded, but not so tightly packed that it puts pressure on the guitar, inside it's case. Scrap pieces of foam rubber--the sort of stuff that's used by upholstery shops--is good for this. But if you've got something very valuable, there are companies that ship nothing but valuables and artwork. They're expensive but quite good for certain items such as oil paintings, sculpture, etc.