Yes, there are differences in practice between driving and flying. This is the first I've heard of a $5 fine and it may just be a new scam. Of course, one would be foolish to present an old FMT on re-entering Mexico; especially one that had expired. Regardless of what you are told by customs officers, who are notoriously ignorant of the actual laws, you are very foolish to ever drive out of Mexico without having aduana remove your car sticker; there are horror stories of those who have had a car stolen, etc. and tried to bring in another. It gets very complicated and very expensive. True, it 'can be done' but it isn't legal and it isn't wise. Read the law yourself.