Early on one of our commercial pilot volunteers wrote a very informative guideline for anyone shipping animals commercially. Here is the link:http://pilotsnpaws.org/references/commercial-transport/
It is under Resources on the homepage of our website.
It is important to visit or call the airline/s you are considering to see what their guidelines are. Unless the animal is traveling in the cabin under the seat with a guardian, there are strict outside temperature guidelines they follow when permitting animals on board or not. This is due to the temps on the tarmac, not the temps in the climate controlled cargo area. Choosing a direct flight, even if you have to drive a distance is preferable as well.
Best of luck!