It is not exactly a shop it is a market and there you can find lots of small shops.
What to buy: You should go to "Mercado San Juan" and buy some handy craft. Theay are really cheap and very beautiful. Also in this market you can find men and women clothes and some gadgets. Whe you go to "Mercado San Juan" try to do it as cool as you can and the best time to go is from 11AM to 1 PM.
What to pay: You'll find cheap things but try to make an effort and a good deal before buying something. That is a tradition in this market.
What to buy:
Even at the simplest supermarket you will be able to find some VERY GOOD COFFEE, which is one of the main things produced in the state of Chiapas and in this region in particular. I particularly recommend "La Finca" brand, which turned out to be very good (plus it has a very fair price!). "Gramlich" brand is also awesome, although a bit more expensive.
However, I've been told that there are several other items made of coffee that are great as well, such as liquor, cream, chocolates, etc. I haven't tried them (YET) but I will keep you posted... I'm sure they must be great, because the quality of the coffee produced in this region of my country is exceptionally good.
An extra food-shopping tip: cheese is also very good in this area, especially the "Doble Crema" type.
What to pay: You can pay as little as the equivalent of 3 US Dollars for a small bag of coffee (half a Kilogramme = 1 pound), and up, depending of the brand you buy. The cheese is sold in big pieces and it should be around 5 US dollars per piece.