Did you request the hole be put in your bumper to route the exhaust through?
send the moron the bill with a letter that includes phrases like 'my lawyer said...' & tell him you'll sue.
1) Never lie in a letter you send in the mail regarding legal matters. It's a federal offense. Hard to prove that you didn't talk to a lawyer, but why risk it?
2) No need to say, "Lawyer said" - that can bite you in the butt too. It's not like, "My lawyer said" is a magicians phrase like "al a kazzam!" that will make things all-good. If you insist on taking this tactic anyway, "My legal council" woks better.
3) There's no reason to get combative right off the bat. It can unnecessarily complicate things, eliminate options, and once you've gone down that road, it will be difficult to change direction. it's If you give them a chance to fix the problem, and it does end up going to court, it will look much better for you that you gave them a chance. If you immediately pull both triggers, go elsewhere and just send off the bill with a nasty note, it makes you look like a litigious monkey's behind, and the courts will treat the case accordingly.