[PHXFlyer]

Are you sure the $704 quote you saw on the Mexicana website was for 2 people or per person. You do not say what city you are departing from however for travel to Cancun over the Christmas holiday $704 for 2 people ($352 per person) sounds almost too good to believe. If it is $704 for 2 then is it a web only fare? Some web only fares can't be booked by the regular reservations agents.

Just for kicks I went to Mexicana.com and did a test booking on the dates you indicated. There are three fares displayed. The first is the "Super Saver" (lowest) fare and under the description it reads:

The lowest fare of Internet

This may be an indication that this is a fare only available on the internet for a new ticket purchase. Most airlines offer these "internet only" fares and unfortunately in most cases prior travel credit can't be applied. Since the Swine Flu outbreak was a unique situation I'm really surprised they won't make an exception for this. Did you try to appeal to a supervisor or manager? Unfortunately if you make a complaint in writing the fare you saw could disappear or go up before it gets settled. What you could do is take a screen shot of the fare display screen to use as a reference if you go with a written complaint.

Good luck.