Temperature Packs

I remember reading on some forum these heat/cold packs that would always maintain a certain temperature range due to a combination of exo and endothermic reactions. Though I cannot remember the name of these or find them anywhere. Help me out??

http://www.shipyourreptiles.com/products

They werent normal packs. They would always emit temperatures within a certain range. They were like 10 bucks or something and I remember sellers would charge the buyer for the pack then offer to refund the price if the buyer returned the pack.