Can you register a car by mail?

You have several issues here. If you plan to drive the car in California it must be registered there, but your Military stautus exempts you from certain fees. If you plan to drive the car straight to Florida you may obtain a "One way Trip" permit from the DMV. On arrival in Florida you must register the vehicle there. DMV forms may be downloaded off the DMV website. You may be required to also submit copies of "Proof of Insurance" and Current Smog Certificates,Some States require a Weight Certificate (except Deisel motors) If you bought the car from a Dealer they can handle all these requirements. If you bought from a private Party you may be required to go to the DMV to obtain and pay for the oneway trip permit ($15). Another avenue is CSAA as they offer their Members DMV Registation services, but you may be required to go to there inpection station. Good luck