We took our first 5th into Mexico a few years ago - registered to husband - didn't return the permit - and then sold it in Canada. We now have a different 5th, and are returning to Mexico in January ...
On line is the only way to go-same price, fast, easy. It will be fine to get your permit for the new one in just one name, and in the name of whomever didn't have the permit that wasn't turned in.