Wow, a lot of you are making negative assumptions about the OP here. He simply asked about the least costly method to ship the guitar, and his mentioning the price of $9k would indicate he also wants the service to be reliable. Some of you are reacting as if he were sending it on a raft across the ocean.

USPS is definitely the cheapest route. If you look up any of the three main carriers in the U.S. (USPS, UPS, FedEx), you're going to find horror stories for every one with guitars. Nothing in life is a guarantee. For very expensive guitars, I prefer to overnight them to reduce handling time. And definitely purchase insurance.