USPS is cheap, but i've had mixed experiences with their delivery methods, as well as handling of packages. It is different from area to area, but you may want to check Fedex as well.
If whatever you're shipping is packed precisely, you can also get away with good shipping rates.
A few key things i'd try to keep in mind while trying to keep costs down are :
- Don't use a box that's too big. If you find a box where the guitar barely fits, and has maybe an inch or two from the top, that's fine.
- Don't have the guitar, or case actually resting along any of the boxes edges. Make sure there is sufficient padding on the bottom, sides, and even the top. The idea is to have the guitar or case be suspended in the box.
Fractal Audio Axe FX