Take this information as you may. This was my experience and I just want to warn you.
Travelocity and Mexicana both confirmed the flights months before I left. At one point, Travelocity wrote to say that the flight from San Francisco to Mexico City had changed and wouldn't arrive in time to make the connection from Mexico City to Chiapas. I called and they were very concerned but said they'd get back to me within three days. After three days, I called and they explained that there was nothing they could do and I would have to call Mexicana myself. I offered alternate dates but they firmly explained I'd have to do the work. I then suggested I cancel the trip since the confirmed dates wouldn't work and surprise, I'd need to call Mexicana for that, too. I understand we are in the rough and rowdy days of the internet and travel agentless travel, but they have a whole marketing campaign about how they are with you before, during and after your trip. They aren't! I won't use them again. There's too much competition to treat people so poorly.
Mexicana it turns out declared bankruptcy and the hold time was absurd, which is what I suspect Travelocity didn't feel like dealing with. I'd wait on hold for 20 minutes, get an agent and explain my case and they'd have to send me somewhere else. This always meant getting hung up on during the transfer so I'd have to call back, explain again and then pray I made it to the right department. I asked for a direct number but there wasn't one. Anyway, I agreed to fly into Mexico City, spend two days and then connect to Tuxtla. On the way back, I did the trip in one day. Tuxtla to Mexico City was fine but then in Mexico City they announced that we'd have to make an unexpected stop in Tijuana to refuel. This doesn't make any sense unless maybe they couldn't buy fuel in the US and needed it for the flight back from San Francisco. The flight meal was packed. Once on, we found out the meal was canceled and the drink service limited. The Duty Free cart came by three times however. The place was an old airbus and I could feel the steel beams right through the cushions and several of my neighbors complained about the same. We then smelled smoke and called a stewardess who insisted no one was smoking. I personally feared the old plane was on fire but she had no such worries as she came by with her personal perfume and sprayed the air so we'd stop complaining! She insisted it was the coffee brewing but she was wrong. I hope it doesn't mean worse for a later flight.
If the Mexicana staff had just said, sorry, it's rough, here's an extra drink or 100 miles on your frequent flyer program, it would have helped. But they didn't and they were kind of nasty and now I'll be flying Aero Mexico, which is a shame because Mexicana was a fine airline as recently as a year ago, slowly going downhill.