Importation of dog into Mexico

We have been crossing the border from US to Mexico at least 3 times a year for the past 17 years and have never been asked for any papers for our dog. But you never know when a border official might ask. The law says you have to have proof of rabies vaccination, distemper etc. and a health certificate within 3 days of entry showing the animal is in good health. Since getting a certificate is very costly we just make one up on the computer but I am not suggesting you do this. We usually cross at the Columbia International Bridge near Laredo. Fast and easy there. But we have used crossings in California, Arizona, New Mexico and Texas. Always the same story. No questions asked. We have the same experience reentering the USA and have never been asked for anything there either. All these experiences are for land travel. Air travel is much stricter.