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Repair shop is required to get your approval
You must give the repair shop your OK before any repair work can be done to your car. You can do this verbally or in writing. Even if you don’t ask for a written estimate, the shop needs your OK before they can work on your car. If the repair shop does any work on your car without your OK, you do not have to pay for that work. Furthermore, if you claim that the shop did work which was unauthorized by you, then the shop has to prove that you did authorize the work. This is why most repair shops will ask you to sign the repair order before the work begins.
When you go to pick up your car after the work has been done, you should get an invoice which tells you all the work which was done on your car. The invoice should also tell you the price of parts and labor, the hours the mechanic worked on the car, whether any used or reconditioned parts were used in the repair, and what the shop’s guarantee is.
The repair shop is required by law to have a sign measuring at least six square feet which informs you of these rights.

What if repairs are made without your approval?
If the repair shop has done work which you did not authorize, you do not have to pay for it. If you tell the shop that they have no right to payment for the unauthorized work, the shop will almost always dispute that with you. New Hampshire law also says, however, that the repair shop has a “lien” for unpaid parts and labor on your vehicle. That means that the repair shop may refuse to return the car to you until you pay.
A responsible repair shop will give you back your car if you pay for the authorized repairs and place the amount for the disputed repairs into an escrow account where it will be held until the dispute is cleared up.
If the shop forces you to pay the disputed amount before returning your car to you and you need the car back right away, you will probably have to pay the shop for the unauthorized repairs. However, you can then sue the repair shop in small claims court to get your money back.
If you do not need the car back immediately, you can go to court and file a claim against the repair shop. Once you charge that the shop has done work without your OK, the shop has to prove that you did authorize the repairs. If the shop cannot do that, the court should rule that the shop has no lien against your car and that the shop has to give you your money back.