You'd need an affadavid from the shop that fixed it for openers or bring them to court with you. This is what I'd do:
The old parts would help but whose to say they came off THAT car. I'd get the old parts though. Should have took pictures before you did anything to the vehicle.
I'd try and settle this thing reasonably first unless they insist on going to court. I'd work up what they owe me and send them a bill certified mail with a letter. Demand a response and have a default response if they elect to ignore you. You may have to threaten and be prepared to use legal action. Sounds like a small claims deal if it goes that far.
**Disclaimer: Any advise here is soley public forum chatter. I in no way am legally advising you to do or not do anything. Thats as far as I go.