Hey Vonnie always have to create problems geesh, LOL 
Heat packs, are mainly used for shipping reptiles when the weather is cold.
** Like I'm getting a NEW friend TODAY, heheheh and he will arrive in a box with a heat pack taped to the inside. Temps in my neck of the woods have dropped drastically lately. We were in the 30's last night.
Oh hand warmers just do not last that long. In an emergency as I said, you may lose power for days. The heat packs could get ya by, until you could provide other means of heat, by moving them.
Anyway here is a nice description of them from Clay Davenports shipping site.
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Hot and cold packs
The need for a hot or cold pack will depend on the temperatures the package will be exposed to on its way to the destination. As a general rule, if the temperatures are between 65 and 80°F I normally don't use either. If the temperature exceeds this range I use the approrpiate pack to help keep the proper temps inside the box.
When using either of these, it is important not to let them lay in direct contact with the container holding the reptile. They are there to change the air temperature inside the box, not to heat or cool the animal directly.
Heat packs are chemically activated and can only be used once. When the pack is exposed to the air and shaken, it reacts with the oxygen to generate heat. Nothing less than a 35-40 hour heat pack should be used. The hand warmers sold at Wal Mart are not adequate for this purpose and should not be considered at all. I order my heat packs from RLD Enterprises at http://rldtheheatsource.com.
To use a heat pack remove it from the plastic bag and shake it very well for a minute or so, then tape it to the inside of the lid of the box. Remember, they require oxygen to create the heat, so only tape the edges enough to secure them, but do not cover them so they cannot get the needed air. Some heat packs have one perforated side, and one side that is not perforated. The perforated side allows exposure to oxygen, so this side should always face into the box, not against it.
Generally only 30 and 40 hour heat packs are used to ship reptiles. There are differences in the two. A 30 hour heat pack will reach a surface temperature of about 115° at it's peak. It takes 20-30 minutes to activate, and peaks betwen 12 and 14 hours. A 40 hour pack takes 40-50 minutes to fully activate and will peak at 110°or so at 17-19 hours.
Depending on the size of the box, or the severity of the weather, more than one heat pack may be necessary, use your own judgement. In a 12x12x12 box, one heat pack will raise the temperature approximately 12-16 degrees, and each additional pack will add another 10 degrees. This will vary to an extent with the quality of the insulation of the box and the severity of the outside temperatures. Insulate very well in winter.
Cold packs are reusable. You can get these at any Wal Mart, in the cooler section. They are normally used in coolers to keep drinks or food cold.
When using a cold pack, I pack the animal, but leave the lid off the box. I put the pack in when the driver arrives so that it has the maximum time to remain frozen. It's a good idea to wrap the cold pack in a paper towel to soak up any condensation given off as it thaws. Again, place the pack so that it cannot come in direct contact with the container holding the reptile.
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Eve / PHEve
