I have seen what DooLittle describes. In fact, the snake that just came in yesterday had the heat pack on the side wrapped in newspaper.
When I ship, I tape the heat pack to the lid with one piece of tape on each side of the heat pack. I also put a layer of packing paper on top of the snake in a folded square (so it looks nice when you remove the top piece of styrofoam) and if the heat pack were to fall, it would fall on that piece of packing paper and never touch the snake anyway.