Tequila: First of all that is "sneaky" posting a link to your commercial web site. No web site that is not an official Mexican Aduana site is guaranteed to be 100 % factual. Mexico Mike is often incorrect. Heck even using information from an official website is not always correct! Depending on the situation that letter from the RCMP may not work. I went through hell cancelling my 10 year permit. I was able to call Mexico city and they insisted on paperwork that I did not have. But another forum member gave me some additional information and I was able to cancel the permit. My process started in Mexico in February and finally while I was in Canada this summer it was cancelled. Cost me a small fortune in DHL fees. I have some phone numbers that may help. From the USA or Canada call 1-877-448-8728. When prompted enter 7-2-2-1-1. The connections are very poor and I was never able to get anyone who spoke English. They are in one of those concrete rooms that sounds like talking in a barrel with lots of background noise From Mexico the number is 01-800-463-6728. But even with my often bad Spanish I was able to understand their requirements. Bottom line is that if you can go to the border do so! Sending paperwork to Aduana is risky. My first package could not be delivered. The office had moved and the person whose name was on the official Mexican website was now working elsewhere. The officials would not accept the package. I was lucky as Estafeta was the courier. The driver called me from Mexico City explaining what happened and he sent it all back to Hermosillo. I resent it and while in Canada they actually called me asking for further information. Much easier to just go to the border. I might also add that Turtle Toad's sticky has the best information available. No need to send people to some other website!
Moisheh