If you are on the return half of your airline ticket purchased in the US or elsewhere not Belize, it has usually been included. If you are on the first leg of a return,or it is a one way ticket out purchased from Belize it is usually not included.
Just a basic rule of thumb but not all airlines work the same way so in some cases I could be wrong. You have to pay exit tax when crossing land borders too.
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We came to the states for our last adventure before we die but as we didn't die yet, there is time for another adventure this time back to Belize.