When I worked in an instrument shop we used UPS, who I would always happily reccomend, we sent at least a couple of dozen guitars a day - plus all the other stuff - and in the 4 years of me working there I know of maybe half half a dozen guitars that got damaged that weren't 100% the fault of the packer.
Course, because we had an account with them we had more insurance as standard plus I guess were in a position to claim any loses against the business or whatever, which you wont have the luxury of, but I still use them if I have to send anything with confidence.