The correct situation will depend on your airline. If arriving at Cancun on a scheduled airline (such as BA or Virgin) all Mexican taxes are included in the cost of your ticket. If arriving on Thomas Cook or Thomsons these are excluded and are normally paid separately on departure at Cancun. In the former case there is no fee due when exiting into Belize but in the latter case you should pay something like 305 pesos for the FMM which you already have (also known as a tourist card). Now, even if you HAVE paid the taxes (e.g. if you traveled into Cancun by BA) some border officials have tried extracting the 305 pesos from you. There's not much you can do other than stand firm with some form of tax breakdown from the airline (usually available online) that clearly shows you have paid.
I will look at the documentation from Air Canada and see what's what.
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