Shop owner's liability insurance should cover this. That is the purpose of having that type of insurance. This shop owner sounds very shady, probably doesn't carry the required insurance, or has had so many claims he doesn't want another. Anyone who does exhaust work knows about pipes and bumper clearance, it's taught in mufflers 101.
Take some time and check out the business owner before you proceed. See if they are worth anything, or are just a fly by nite outfit that is on the brink of folding. Check BBB for previous complaints, check local courthourse or judicial website for lawsuits filed by them or against them. Get on the internet and google them, find out as much as possible. It may not be worth going after them, you can't get blood from a stone.
Don't waste your time with a lawyer, lawyer will cost you more than the damage and you will probably never recover legal fees. If you decide to go after them get estimates to fix the damage, send to the hacker shop and give them 30 days to pay or repair. If they refuse, file a case in small claims court and suit them for the damages, all of your time and aggravation, and your expenses. Don't get car fixed until you have settled the case.