The secret to shipping an instrument is not just good luck, it’s proper packing. Shippers have guidelines for shipping items. If a shipping company suspects inadequate packing they’ll quite likely use that as a way to avoid paying for your damaged instrument, no matter that you paid for insurance or that the shipper accepted a carton that appeared to have been assembled by a kindergartner. So use a sturdy corrugated carton, pack your instrument inside of a good case, and surround the case with at least two inches of packing material when placing the case in the box. When in doubt consult with your shipper.
Here’s an example of a shipment received without any packing material. The box took a real beating. Someone at the shipping company spent a small fortune in packing tape to keep the box together enough to get it to us. Luckily, we were only receiving a shipment of empty cases. Gives you an idea of what can happen, though. Don’t scrimp on the crumpled newspaper, bubblepack, whatever….
We don’t recommend what one fellow did: Putting a lot of duct tape and stamps on the case and simply mailing it. It amazed us that the instrument arrived intact (so don’t pick on your postal carrier), but don’t assume that you’ll be as lucky. You can be sure we won’t be shipping the guitar out the same way.
If you can’t get a shipping carton from a local store, try an appliance store and cut down a refrigerator box, for example. We once shipped an upright bass across the country that way (brought out our inner engineers).