No to hand warmers.
You don't have to drive with the heater on if your snake alone is insulated. Place the snake in a small transportation tub or snake bag/pillow case. Place that into an insulated cooler if you are worried about environmental temps. You can add a hot water bottle in a sock too if you don't want to buy a shipping pack. I've had 16oz water bottles keep a steady 80-85 degrees for 4-5 hours in a insulated foam box when I transported snakes.
And even if you drive at 70-75 degrees, your snake will be fine if it sat uninsulated. How do you think reptiles do at shows in room temp without heat.
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