Sure they can only demand it... but seriously... those are people on the other end of those phones... people with money problems just like us... I would imagine that if you were to offer to pay your debt in full with certified funds and then some extra $ found it's way into the envelope in the form of cash, it might go a long way to softening the hearts of these evil CA employees... I could see how that would certainly be risky for them and could get their asses fired, and I could also see how you might just lose your $ without any benefit, but I can't see how that would be a violation of any federal law, they are after all, not government employees.
In my business greasing the wheels is just part of the game, it's so common if you don't know how to do it you'll get left out in the cold... I have often wondered if we could get this to work in the credit world. A good will letter might carry a lot more weight if it had a portait of Mr. Franklin tucked into it.