The last 2 times I've departed Belize, both times with American Airlines, I was told the tax was included in the ticket. The last departure was Jan 17th. My tickets were frequent flyer tickets which cost less than the former departure tax. I think I came out ahead by about $10 US.
The same thing happened to me and I couldn't figure it out. I didn't put two and two together about using a mileage ticket. I had enough miles for several people to go, so when they lumped them together I thought that it was a "bulk discount." Thanks for connecting the dots for me because my exit tax was $9 US.