You can't just use Hothands or things like that though, because they get too hot and don't stay hot long enough. You need heat packs made to ship animals with.Cabellas had some various heatpacks. Try a camping supply store as well, like Gander Mtn, Tractor Supply, Bass Pro or Cabellas.
Uniheat is all I touch! I packed a psalmo MM in my usual bulletproof insulated box I cut and assemble myself and shipped a NON LAG priority over christmas last week. Shipped on Monday and Arrived on Friday and it was low Teens here for temps.. SURVIVED PERFECTLY!^ exactly. But, I have seen 40 hour heat packs at bass pro. not sure what there actual temps reach...but dont get the "hot hands" brand.
They peak at 140F and drop off very quickly.
I use the uniheat. I bought 100 of them last year from ebay for like 30 bucks.