MyBeautifulAir--You can buy peanut butter and refried beans here. You can always make a mean enchilada sauce yourself, too, and on this forum there is a recommendation for a mexican woman who makes/delivers tortillas (though I'm not sure if they are burrito sized, I miss those too--However! if you go to CBC during the day and speak with one of the owners they may sell their giant tortillas. Last I checked (over a year ago) they were willing to sell them at 2pesos each). Tortilla chips are also available here--just the other night I saw a big bag of white corn tortilla chips at Disco! If you learn to make your own salsa you'll be much better off, but there's also a texan here who is selling homemade salsa that's pretty dang good!
Things to send: Cheddar cheese is a great one, as it's definitely not available here, and other good cheese. I always bring in palm sugar, hot sauce (cholula, tapatío, even tabasco since the markup here is insane), vitamins, hot chili pepper powders of all varieties (chili, cayenne, chipotle), dried chipotle peppers, graham crackers, reese's, chewing gum, dried fruit (like sour cherries, cranberries, craisins, etc.), and coconut (cooking) oil.