Maybe the bill is higher than the estimate. Maybe you were charged for parts that were never installed, or shop hours that don't match how long the car was there, or a diagnostic fee you were told would be waived. "Overcharged" covers a lot of ground, and the first useful step is naming which kind you have.
Over the estimate. There was a written number and the final bill beat it without your approval. This is the cleanest version — see the guide on when a shop charges more than the estimate.
Billed for work not done. Parts listed that aren't on the car, labor hours that exceed book time by a wide margin, a "replaced" component that is visibly the original. This needs a second opinion — another shop's write-up of what is actually installed is worth more than an argument.
Junk fees. Shop-supplies percentages, storage fees you weren't warned about, a diagnostic charge that was quoted as free. Some of these are regulated, some aren't, and it varies by state.
An itemized invoice is the thing to demand first, in writing, if you don't already have one. A shop that won't itemize is telling you something.
Then compare: the estimate to the invoice, the invoice to the car, the labor hours to a published labor guide. Photos of the engine bay, the old parts if you got them back, and a second mechanic's assessment are what move this from "I feel ripped off" to "here is the discrepancy."
You may not have much if: you approved the work and the price when they called, the parts are on the car and the hours are roughly in line with book time, and the fees were printed on the estimate you signed. Feeling that a repair was expensive is not the same as being overcharged, and a shop's prices being higher than the one down the road is not itself a violation.
Write a short demand naming the specific charges you dispute and the amount you want back — not a general complaint. Attach the estimate, the invoice, and any second opinion. If the shop won't adjust it, small claims is the usual path.
Wherewithal helps you organize the paperwork and draft that letter. It is not legal advice.
Wherewithal is an organizational tool only. Not legal advice. Consider consulting a licensed attorney.