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

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Dmitry
90' 300CE
17" Intra Signums
Brulen mufler
Euro Headlights/Clear Blinkers
White gouges (made my own)