I have this problem at the farmer's market stall. I had exactly the same issues; steam in the bag, cookies rock hard and dried out. I personally find that after heat exposure they just don't taste the same. But I never bake plain sugar cookies, I have a lot of different flavor mixes and extra ingredients, etc. So that might affect it. This is a high-humidity area, too. I got a cooler that cools down 20 degrees from the ambient temperature. It can plug into the car outlet. I imagine any protection from the heat would help - no reason a cold pack can't do the trick, especially for a short time. Good luck.