[Advdave]

I cancelled my tvip by returning it to the banjercito office. Email them, a fellow named Gerd has been most helpful. He had me return the permit dhl, when it arrived at banjercito he emailed me when the permit was cancelled and refund issued.
A buddy did the same when his bike broke and he swapped bikes, cancelled one tvip and got another. one has to be cancelled before another can be issued to the same person.

you have to return it before the effective date. if the date of entry into Mexico on the tvip has passed, might be a different story. in any event banjercito folks were very helpful.

good luck.