I got the boxes and the heat packs from the place you used to work at. The boxes were free and the heat packs were $2 each. They are very similar to the hand warmers you can find at camping stores and even Wal-Mart. I don't know about packing foam but newspaper is sure easy to use.
Yes you can use water bottles but they add a lot of weight which will add to the shipping costs and the heat packs, I used , were good for 20 hours. Also who knows how rough they really are with the package; I wouldn't want 2 liters bouncing around a $4,000 snake.