Those cargo cabins on passenger jets are pressurized. In instances where the cabin is not properly pressurized, you end up with a plane-full of upset passengers whose luggage is soaked in about 20 kinds of cologne and perfume (don't ask me how I know this, lol) Not sure exactly what UPS and FedEx do...I'd assume similar conditions, but they are just large cargo planes.
I definitely wouldn't roll the dice and hope for a knowledgeable TSA agent. Being that the price of extra checked luggage is starting to get a bit ridiculous, you may only be saving a small amount of money by not overnighting the package via FedEx. If both options are available...I'd just ship. I know that FedEx and UPS may not have the best reputation for caring about your package...but they generally do pay attention to cautionary notes (live fish, fragile, etc) on packages. Baggage handlers at the airport though? lol good luck.