I think the buying notes idea is good.
You could buy notes from the supplier, 2k for 10 Notes.
then to use I have two ideas.
Either just stick with what we have /note <text>
or the harder to code way,
Use the note (from your inv) (it brings up a screen where you type a message) then click ok.
As for fines on cars, this is a great idea - it could use the same method as the padlocks when it attaches to the car. I can see it being hugely abused though. Also where should the money go?
Divides between the police?