Gary,
My understanding is that when you visit Mexico for less than a week, from Belize, then you don’t have to pay the fee when you leave Mexico. For those who travel from Belize to Mexico, and stay more than a week, there is definitely a fee. I think it’s around $20 US, but will defer to someone who had done this recently.
Of course, you are flying into Cancun and then traveling over the border to Belize, rather than doing a round trip from Belize. We have some Corozal expats on the forum who make that trip on occasion. Hopefully one of them can enlighten us. Also, if we don’t have an expert on this forum then you can try the Belize Forum. I don’t fly out of Cancun anymore. And the fees coming from Mexico to Belize change often, without much explanation.
FYI – WE often have Menzies VIP service pick us up at in Chetumal. We take a bus from Merida and then work out a pick up point. They always know what the latest fees are, since they shuttle people across the border daily. If there is a fee, you won’t be able to avoid it.