I like the idea of just walking past immigration.

We are going to Mexico in about two weeks, staying about 10 days and flying in and out of Cancun. On the airline flight information receipt, it clearly shows that the Mexican tourist tax and the Mexican exit tax were paid as part of the "taxes and fees" with the ticket. Will it help to take copies of this and also copies of the IATA document referenced above to show anyone who tries to get more exit fees out of us? And also hang on to the tourist card/FMM.
Yes, I would keep the airline flight information receipt with your passport & FMM and produce it if asked for money at a border. Of course, they might say it's a fake copy. At the Chetumal border, if you have any trouble, just walk across the road to the real immigration office.
You said there is a "Mexican tourist tax" and "the Mexican exit tax".
What are the amounts?
The tourist tax/fee will be for the FMM and should be 294 pesos or approx. US$20.
There is no departure tax/fee for Mexico.
If you leave Mexico and go to Belize, the official procedure (at least according to the Embassy & Honorary Consul) is to hand in the FMM and get a new one when you re-enter Mexico. You can try asking for a re-entry stamp (doble entrada) and keeping the FMM, but you may have to pay 200 pesos or US$20 to do this (I've heard reports that this has been done).
If you get a new FMM when you re-enter Mexico, you should have to pay the 294 peso fee. It can be paid at a bank or at Cancun airport. I was once asked to pay the FMM fee when I checked in at the airport, but I had already paid the fee at a bank, so just showed the receipt -- no problem. I've also heard that other travellers who did not pay the fee at a bank, also were not asked to pay when they checked in at the airport. Mexico -- anything can happen.
I hope you report back and let us know what happens.

Mister Bee and all: Delta Airlines is charging me $USD 21.80 on code UK for "Mexican tourism tax" and $USD 27.88 on code XD for "exit tax." Clearly not doing me any favors with the exchange rate. Not large amounts, so I am not going to fuss about it, but I am going to protest any effort to get more from me.
At today's rate, US$21.80 is MX$287 and US$27.88 is MX$367.
The first fee looks like the fee for the FMM tourist card (actual fee is MX$294).
The second fee, I don't know.
If you want an answer, you'll have to ask the agent or airline.
Perhaps there is an airport fee? Cancun airport or the US airport??
Is there a departure tax from the USA (I'm not an American)?
I can only repeat what the Embassy & Consul have told me, "There is no departure tax from Mexico".
I saw the Honorary Consul again today, and was told the same things that I have already stated in previous posts.
I did get an answer to one question that I had concerning the collection of the fee by airlines and travel agents. According to the new rules since 1/1/2012, airlines and travel agents are "no longer authorized" to collect the fee. That had already been announced in a press release issued by the Embassy of Mexico in Belize.
Me: What about travellers who have bought tickets since 1/1/2012 and have paid the fee to an airline or travel agent?
Answer: In that case, the traveller should carry the proof of payment and show it if asked to pay money at a border.
Me: But the immigration officials at the border should not be collecting any money anyway.
Answer: That's correct, but it's Mexico. Any money collected by the border officials collect will go straight into their pockets. The tourist should know what he has to pay. There is a bank at the Customs building.
So, what it boils down to is every person crossing the border should have evidence that the fee has been paid. If you think the fee has been collected by the airline, you should get it in writing and carry it with you to avoid any hassles at the border.
If the border official asks for money, walk across the road to the large immigration building.

Yikes, is this confusing or what? OK then, just to confirm please. As of today (25/Apr/2012), can someone categorically confirm the following please.
a) departure tax (if any) when flying from Mexico City to Havana
b) departure tax (if any) when flying from Cancun to Havana
c) departure tax (if any) when leaving Mexico by land into Guatemala
d) when leaving Mexico by land or plane (and being in Mexico for less than 6 months), isn't it necessary to return the tourist card? That being so, isn't it then required to buy another Mexican tourist card if returning to Mexico by land? Would that still apply if one was in Mexico for less than 6 months on the original tourist card?
Or, if you've been in Mexico for less than 6 months and tell the exit immigration officer that you're coming back to Mexico shortly, would you be allowed to keep the tourist card and then simply use it again when returning to Mexico?
p.s. does anyone know why the Mexican Government doesn't do something to stop these scams? Surely they must know????

#17, when did they change the "double entry" rule? It used to be valid for 6 months come and go as you please.
On 1 May 2010, Mexico’s National Immigration Institute implemented some changes to its immigration procedures. The FMT entry form for tourists was replaced by a new form called Forma Migratoria Múltiple (FMM). According to statements issued by the Mexican Embassy and an Honorary Consul, the double entry is no longer permitted with an FMM. However, that doesn't mean that double entry stamps are no longer given!