Assuming you are shipping in a standard fish styro (17" x 17" x 10") I would use a couple gel packs or water bottles, warmed of course, along with one or possibly even two heat packs given current weather conditions. You may also want to keep an eye on the weather at your destination as well, as this is where problems sometimes arrise. I won't ship is the destination is below -10 C.
You don't want the heat pack to be able to come in contact with your snake, but placing them in the corner(s) of the styro on the hot water bottles or gel packs will keep the water/gel volume warm longer. I would then put a layer of crumpled newspaper around the heat packs and water bottle so that the snake can't come into contact with them.
Also, I'd suggest removing the styro from the cardboard box and puncturing a couple holes, no bigger than a pen. Then pop the same size holes in a coule sides of the cardboard as well, not lined up with the holes int he styro. This will allow for just enough ventalation to feed oxygen to the heat pack(s) without suffocating your snake.
I would also suggest you confirm with the air line when you are dropping off the package that the flight does indeed have heated cargo. Sometimes in winter flights get bumped and the plane may have changed from the time of booking. It is possible to safely package reptiles for a nonheated flight, but I wouldn't suggest it if this can be avoided.
That should be about enough babble from me