In my experience, a lot depends on the FedEx employee accepting the package. I had an employee tell me about the 500.00 opening-the-package limit (I insured that one for 499.00 at his suggestion), and then the next time I went to the counter, I mentioned this and another guy on duty waved his hand in the air and said, "AAAH. Don't worry about it."
Also, if you read the next paragraph in the regs cited by DaveK under the "ground" ruling, it will state that a firearm is accepted for shipping from an individual to an FFL for the purposes of repair and/or customizing.
