I shipped my Nile and 2 geckos from San Diego to Pittsburgh Pa in January. It was frigid cold in Pittsburgh (about 2' snow the day they arrived), but they had sufficient heat packs and were well insulated.
I wasnt sure as to how to ship a monitor (didnt have internet access at the time and getting suggestions in person from LLL reptiles Esco crew was the best I could do), but this is how I shipped him:
I used a good sized rubbermaid tub (about 15" long x 10" wide x 5" deep) filled with shredded paper towels, to provide something like a burrowable substrate and insulate abit. The monitor was bagged and placed into this tub (Note: My monitor liked to use the bag later on as a kind of hidespot). I punched a number of air holes in this to provide some ventilation. On one side, I placed 3-4 heat pads (as they simply refused to warm up well for me)- 2 attached to the sides of the tub and 2 on the bottom. The lid to the tub was taped on and it was placed in a larger shipping box (i think it was like 18' x 14' x 12 or something) and surrounded with crumbled newpaper, then a blanket was set all the way around the tub as a final line of insulation (like i say below, I forgot to insulate the bottom
). This was taped up, a few small vent holes punched and marked with "HARMLESS LIVE REPTILES -HANDLE WITH CARE" in large black-marker letters on each side/top of the box.
Along with providing temperature insulation, you also wanna provide what i think of as "slam" insulation- that is, just incase the person handling your package isnt quite as, uh, "gentle" as they should be, you dont want your animal getting battered about.
Also, something I did wrong that you should be careful of:
Dummy that I am, I forgot to insulated the bottom of the box. Well, you can imagine how worried I was when my lizard arrived and the bottom of his box, right where he was and which had been sitting on the cold metal floor of the truck, was absolutely freezing. I opened it up and he was ok, but still, beginner mistake that you wouldnt wanna make yourself.
Dont know if thats proper, but Im sure someone will correct me if my method was failing.
Good luck!