Drive to the nearest border crossing. Do it right and you won't have to worry about it. I canceled many permits for people. If you decide to attempt it by sending the paperwork via carrier and following up by phone, you will need to be fluent in Spanish for starters, able to understand a myriad of phone options in Spanish and then constant follow up for at least two months. I did five of these cancellations for other people.
Remember that a "receipt signature" from FedX or DHL, only says it was received at the address. Finding someone from there with the information if it hasn't been logged in yet can be difficult.
That's why I say, to it right and do it the easy way.