Tropic charges up to three seats (three seats for a 70 lb dog)
and Maya charges only one. I do recommend the water taxi if you have time, it's much easier on the dog. Once they get out in Belize City it is so hard to get them back in for another flight. My dog truly enjoys the breeze on the water taxi, too.
BE PREPARED with all documents and papers in advance for BAHA and have your copies when you leave the states, even though BAHA will tell you they are waiting for you upon arrival to BZE. I had an incident once where I was denied my US flight.