How cold is to cold to ship?

my only DOA was a baby boa shipped from CA to.. I think it was Ohio? or PA? from a failed heat pack, boa showed up super cold and dead. This was back in '03, I don't remember what the temps were but it was winter. I think lows 40s? I no longer ship anywhere if temps are lower than 60. As I understand it the heat packs generally raise the temps in an insulated box about 10 degrees.

I look at several variables:

1.) My overnight temps.
2.) Overnight temps at the destination
3.) How close to holidays I am shipping
4.) Species involved

As a "for instance"; I have shipped wood frogs when it was 10 degrees out, no heat pack at all. Why? They're wood frogs, and can freeze solid and be just fine. Conversely, I have shipped wood frogs to NM when it was 90 out, and included about 5 cold packs. Again, arrived just fine. Would I go no heat pack for tropicals in extremely cold conditions? Of course not. As a matter of fact, the lowest I will ship tropicals is around 35, and that is of course with at least one heat pack (depending on destination). In other words, the temp range described by SYR is a very generic range that works well for many species and destinations, but is not the gospel.