Hi IndyGuy,
My papillion (9.5 lbs)DeeOGee made 18 round trips to Belize with me, over a 3 year span. We flew US Air all but one trip. I never had a problem with the airlines at all regarding the trip. She fit in a large carry-on, which was approved by the airline, however, they would ALWAYS tell me that it didn't fit completely under the seat in front of me. So, I would carry a blanket, and cover up my legs, thus covering the case until we were off the ground. My vet would give me "doggie downer" pills and she would get one the morning we would travel.
One word of advise, make sure you have copies of your permit to bring Roscoe into Belize!! Just because you have paid you $, and had conversations, don't think they will absolutely have your paperwork. Out of DeeOGee's 18 trips, we had to pay a "fine" when they couldn't "find our paperwork", over half the time. It actually got to be a joke, I'd just walk in with the $ out already, and tell them to just let us go instead of looking for the paperwork.
That said, it has been 2 years since I last flew in with her, so maybe it has changed.
Good luck with your BZ stay, and kisses to Roscoe.