There are steps you should take to protect your hermies when purchasing them in cold weather. Here is what I do to keep them warm, feel free to share your tips also.
I keep a KK with sand and saran wrap and a mister in the car with me. I big warm towel or blanket to wrap around the KK. I take these into the store with me and put the crabs right into the KK and bundle them up. Do this before you get to the register if its near the door where cold air comes in. Then I have Matt bring the blazer right up to the door to pick me up with the heater on high to make sure the vehicle is warm too. Once we are settled and the vehicle is warm, I will open the KK to lightly mist the air and then reseal it.
Purchasing crabbies is our LAST stop so that we go straight home.
Once we get home I bundle the KK up in the towel again and hurry into the house. I place them in a warm room if we have other items to unload.
Something I do that has nothing to do with the weather is having a ISO tank prepared with the humidity and temp at 70. I slowly ramp the heat and humidity up over the next few weeks, helping the crabs aclimate without shocking them. Then they move into the regular tank.
Cold air can be very harmful to your crabs, even if its just briefly so do your best to keep them warm until you get them home.