Here is a ebay link for 40 hour heat packs.
Many people ship stuff with 20 -24 hour heat packs and the package sometimes gets delayed.
In cold weather, this can mean the death of most corals unless you use a longer rated heat pack.
You can get Uniheat packs in 40-48 and 72 hour heat packs.
Also remember, the heat pack is activated with oxygen. So if you seal up the container,,, as soon as the heat pack uses up the oxygen in the package, it will go out and the corals will get cold and die.
http://cgi.ebay.com/10-pieces-40-hou...12250813a11051