PERMITS
Items or materials that are deemed restricted or controlled by the Mexican government will require special permits before they can be crossed into México. Your broker can apply for the permits on your behalf. Most permits are issued in México City. Certain Mexican customs regulations vary with time, and what may or may not be a restricted item one day, may change the next day or in a week or in a month. There is not one book you can get that accurately lists which items require permits.
Some permits take several weeks to secure. At the time the permit is requested, you must know the amount of the materials you wish to import; as the permits will only be good for that specific amount. They are not designed to be open-ended.