I just saw this thread, so I am late for this particular question, but I can still offer some winter shipping insight.
The ShipYourReptiles "no-ship" periods apply to holidays, typically. As Harald mentioned, shipping right before a holiday can be troublesome if there is any kind of delay, as transportation may not resume for another 3-5 days.
In the case of the Christmas holiday, we started our no-ship period early in December this year. Package volume for gifts and such just gets so heavy, delays, misroutes and exceptions increase significantly, and that makes it problematic and unpredictable for shipping live reptiles. Better to wait for the SYR no-ship period to pass, and then find a workable day for your shipment.
The SYR Shipping Standards allow shipping of live reptiles down to 38F for a daytime high in both origin and destination. We want shippers to have the best chance of success, so our temp guidelines are pretty conservative.
Plenty of breeders ship below 38F as a daytime high. As Pro Exotics Reptiles, I would ship into the teens with little trouble. But that takes experience, and some subtleties of packaging, and at ShipYourReptiles, we want everyone to expect success, so 38F is the cut off.
You can find our high and low temperature guidelines here.
Whenever you are shipping in cold weather, insulation is your friend. We see more trouble and DOAs from the OVERUSE of heat packs and/or the use of hand warmers than anything else.
In a 15x11x7 box or smaller, which covers 90% of reptile shipments, one 40+ hour heat pack is all you need. To better your chance of success, double your insulation instead.
Double up your 3/4" insulation panels on all sides. Or get some 1.5" foam core from Home Depot and cut your own panels. Or consider using one of the one piece insulated boxes.
You don't want your box to get HOT during the winter (as it will with more heat packs), you just don't want it getting COLD. You want it to stay temperate. Increasing the insulation value and effectiveness will accomplish that.
The other common technique was also mentioned above. Drop your package off late in the day at the actual FedEx facility (but before cutoff) and have it shipped Hold for Pickup to a FedEx facility. That will keep the package from being exposed to extreme temps (summer or winter) on trucks and during delivery.
If you are shipping during a holiday, or extreme weather hot or cold, feel free to contact us directly. We can walk you through making the best choice, we do it all the time : )