Shipping in extreme low temps can be challenging, but it is actually easier than shipping in extreme high temps. With extra low temps, you don't want to use multiple heat packs (in a typical 12x9x6 size box), but rather look to double up insulation.
Using multiple heat packs can easily overheat your box and cause DOAs. And using hand warmers, which peak at 180F+, but have an incredibly short duration, are very simply a recipe for disaster.
We recently put together a chart showing the temps and durations of shipping heat packs, the 60 hr and 72 hr packs look pretty appealing!
