MEMBER "RDWSP" DID NOT FINISH PAYING BILL

Go to a Judge with the bill and file a lien on the car.
Some states the owner can take the car without paying the bill and some they can't.

Example:
CA you can keep the car until the bill is paid, but that doesn't apply in AZ.
In AZ the legal owner can show up with a Sheriff and take the car bill paid or not.
Then the shop must sue for the bill and hope they get it.

This is right off a page showing Arkansas lien laws.
"5) An Arkansas Mechanics Lien Can Only Sometimes Include Profits
An Arkansas mechanics lien claim must be stripped to simply the costs of the labor and/or materials furnished. Any and all profits cannot be included in the lien claim except under one circumstance: You are engaged in a fixed price contract and you have completed the contract."

From this page.
Arkansas Mechanics Lien Law: FAQs and Free Forms