I buy sheets of insulation. Either foam board or 1/2 inch-3/4 inch styrofoam. I cut it to size (so the pieces fit tight) and line the boxes. Any time of year, this insulates the snakes from sudden temperature changes. I've tested it out in 90 degree heat. It takes many hours for the inside to warm up to 90. Same thing when I put them in the freezer. Takes a long time for the inside to get too cold.

Then the snakes are placed in hard plastic containers (melt small air holes in it and line with paper towel) that are small enough that the snakes are cramped in there. This prevents them from bouncing around and getting injured. Then wadded newspaper keeps the container from bouncing round inside the box. Whatever you do, don't use flexible containers, containers that can shatter, and don't forget to tape the lid shut securely. DO NOT poke holes in the box or insulation. It's not necessary. I really need to do a video tutorial on snake packing. Proper packing is critical if you want them to get there alive. NEVER use those air-activated heat packs. They will consume all the oxygen inside the box.

As long as the temperatures are expected to be above freezing at the destination city, or cooler than 90 in the summer, you should be OK. The rest of the trip they will be in climate controlled buildings or aircraft. That's using USPS EXPRESS mail though. Can't vouch for any other method.