Shipping dart frogs with heat packs

Hi, I couldn't find the info I need searching, so...

I'm trying to get a reliable packing method down for shipping dart frogs in cold weather.
I've been doing tests in the garage (temps 20-38F), lately I've been trying a cubic foot box, insulated with 1/4" reflective bubble wrap from menards, in which I put the styro cooler that would contain the frogs in their cups.
The internal dimensions of the cooler are about 8x4x6", and the walls are 1.5" thick. I tried using 3-40 hr heat packs from superior ent. taped to the outsides of the cooler, then packing the space between with crumpled newspaper.
I've tried it with the seams sealed shut, then after reading this, tried just taping the box shut, leaving the side seams un-sealed, with pretty much the same results, I open the cooler 24 hour later, to find the inside of the cooler only about 5-10F warmer than the outside temp.
The packs are cold and hard, then when I bring them in the house, the packs start to fire up again.
What is happening? Are my packs running out of air? Or is my outside box not insulated good enough, causing the packs to freeze at some point?
Thanks in advance,
Brian