I don't think they will bother to respond or try to get their money.
3 years ago I stoped payment for slopy 60k mile service at a dealership. Sent them a letter explaining why. They never responded, sent me the same bill once every few month for a year and than stoped.
Never heart from them again
17" Intra Signums
Euro Headlights/Clear Blinkers
White gouges (made my own)