Despite the momentary satisfaction you get from being sarcastic this way, there's no real reason to denigrate a person who's about to write you a check. Also, you should know by now that people learn different ways, and you might have to teach someone using more than one method before they can internalize even the most obvious -- to us -- thing.

Don't be snarky with clients who don't get it. Maybe they'll get it from someone else next time.