Beware that once you get the lawyers involved the issue will take on a life of it's own.
Unless there is "damage" under law as a direct result of the servicing companies actions, I wouldn't even consider getting into a legal stoush. Even if you win, Legal fees will absorb the costs.
I'm sad to hear that A&Ps over in the US are similar to the one's here in Australia. They sem to charge a lot for stamping books and do little actual work. On top of that the most dangerous flight is the one from the annual. I had a massive top end oil leak and lost boilts from the rocker covers right out of an annual! They forget to re-connect the landing light, leave bags of bolts hanging off the wings, leave access panels on the wet grass!
Make sure you have a written agreement for EVERY bit of work they do.
I've had attempted massive overcharges for extra work at nearly every annual. Unless you agree to the work, you DON"T have to pay for it. (that's why car dealers have you sign a work order over here when you drop your car off for service).
If a workshop attempts to charge for extra work, get a second opinion.
I had one shop state that the aileron rod ends had too much play and needed replacement. The tech stated that "you don't want to get flutter" and tried to add an emotional requirement to the work. Thanks to forum members I found out that the play in the ailerons was average. And there's only been ONE Mooney that broke up in flight due to flutter (and that was from the elevator in a horrendously fast descent). BTW the shop didn't even have their own rigging board for the ailerons either!
You don't have to pay for unauthorised work and the shop cannot keep your aircraft either.
I ALWAYS make sure the quote is in writing and all communication is in writing regarding your dispute. (Emails are legally valide here) and if the shop gets legal advice on the issue the advice will come back that they had to get authorisation.
I'm ALWAYS ready for the dud now.
Negative feedback is so hard to give, yet so valuable to a business.
Because the safety issues involving aircraft are so emotive, some unscrupulous persons think they can oversharge for work and overwork the aircraft. I have lost count of the number of shops that want to tear the interior out EVERY YEAR for the cage inspection.
And I just got off the phone with a mate who just found out the automotive shop that sold him his fuel pump took the regulator that was asupposed to be included out of the kit