I ship during the winter, and only avoid the coldest periods (below 0 F) temperatures. I use 40 hour heat packs, and I use enough of them to compensate for the weather--in MOST cases, only one is necessary for a 12 X 9 X 6 box.
I buy the heat packs and insulated boxes from SYR or directly from Superior Enterprise.
If a package is delayed one day, there is still enough heat to get them safely to their destination. 60 and 72 hour heat packs are also available, for those who are especially paranoid.

Important tips: pay attention to the temperatures both where you are shipping from, and at the destination. Put off shipping if the variation is so extreme that an animal might overheat with heat packs at one end, and freeze at the other.

Mark the boxes correctly as containing live animals, and advise on the labeling that they be kept out of extreme temperatures and sunlight, and not placed on truck floors (which can get hot).

Ship no later in the week than Wednesday, to avoid a box potentially sitting somewhere over a weekend.

I personally would have no problem at all shipping from FL to NE right now. I would use a 40 hour heat pack, and drop the box off at the UPS hub--I use ShipYourReptiles.com (or Fed Ex hub, if you have a reptile shipping cert with them). Drop it off later in the day, about 5 PM, so that it won't sit in the office very long. So long as the temps in FL aren't above the low 80s, it should be safe to ship them from there. They'll go out around sunset, and they'll be on a plane before they are likely to overheat.