If I am at my LFS during the winter, I will have a coat and probably a sweater to stay warm while I'm outside. I travel in a heated car so the extremes really only need to be dealt with for a few seconds. Usually, if I pick up new fish, I just close my coat around the bag of fish and me. My body helps keep the inside of the coat warm and the short exposure before you are in the car means the bag of water and fish never really changes temperature during the trip. I leave them inside my coat until I am in my home so they also are not exposed to extreme heat by being the wrong place in the car. The fish don't know any different and I don't mind looking a little silly to get them home alive.