At least for my trip, checking through Orbitz, the best deal was Avis with a midsize/standard rental at $282. However, I was able to get the same size class also with Avis through the AAA website for $222. No Avis discount codes seems to work in Cancun during the Christmas vacation season and most of the coupons seem to be valid only in USA/Canada/Puerto Rico.
You need to have a valid AAA car number to reserve through the AAA website.