If you purchase something in Mexico, you have an advocate in case of problems.
Firstly, always get a receipt. If you can have the merchant sign it where it specifically states what the receipt is for, you will be protected if you ever have to go to court.
The Federal Agency of Consumer Affairs is an agency that promotes and protects consumer's rights. They can provide assistance with problems related to retail and services... Hotels, travel agencies, restaurants, transportation etc.
When purchasing a product or hiring a service, you must assure yourself that it is in good condition and you should request a receipt, invoice or purchase order that proves your payment. If you wish to file a complaint during your stay you can call: Tijuana: 664-636-7000 & 08.10,15.
Mexicali: 686 577-3084 & 85
Ensenada: 646-178-8573
You can email: tijuana@profeco.gob.mx