In the summer you often have a pet embargo from May to September with several of the airlines. The will refuse to ship pets due to the extreme heat thus an embargo is put in place. Continental airlines will still ship during this time because your pet is never left outside in the extreme heat. With Continental airlines the pet is transported from the airplane to the terminal in air conditioned vehicles so they are not exposed to the heat for any length of time.
Winter transport can be very tricky too because the airlines will not accept a pet if the temperature is going to be under 32 degrees at any point of their travel. This makes it rather difficult to get dogs/cats to the northern states during the winter months. As in the summer we have to watch the extended forecasts and hope for a heat wave.
If you live in the US you can get a pet acclimation certificate from your veterinarian showing that your pet can withstand temperatures to 22 degrees. This is not an option when living in Costa Rica as the vets here will not issue the pet acclimation certification. Most have not even heard of it. Think about it, why would they need to living in this perfect climate year round.