I wouldn't use the hand warmer packs that are made for human use. They get pretty hot (IIRC up to 140?) and don't last very long. I use 40 hr heat packs: http://superiorenterprise.com/produc...roducts_id=229

These are designed around shipping animals. They last 40 hrs which is perfect in case there's some problem or delay in the shipment.

If temperatures are above freezing on both ends then I'm usually comfortable with shipping. If they're below that, or if there's a looming storm, etc that could seriously foul up the shipping (plane delays, etc), then I don't risk it.