Heat packs?

[slidewithme]

I have a few people that are part of our Reddit sub that I want to send some s. minima to, but it's 40 degrees here in California. Most of them are in the midwest, one in the middle east coast. In order for the plants to arrive alive, I'd like to include some heat packs.

The ones I've found for hunters/fishers/golfers etc only last for about 10-18 hours. Are these the ones typically used? I'd like to buy them in bulk if possible; I ship plants about once a month and could some for this and next winter.