I would continue to let the shop attempt to fix the problem if you believe they are sincere. It has been my experience that "you catch more flies with honey, than you do with vinegar." Continue to act in good faith to allow them to fix the problem while continuing to document each attempt and failure. Also, review the warranty language and bring that to the prompt attention of the shop. Another avenue is to write a letter to the local Better Business Bureau and send a copy to the shop. If all else fails you may need to seek representation in your local area to proceed in Parish Court (small claims).
That's entirely up to you. If a business is trying to fix a problem and they're in good faith and you've got a reasonable belief that they can achieve the desired result, then give them as much time and as many tries as they need. But if all of those things aren't true, then don't waste your time giving them more time/tries.
You may be bound by a written sales agreement, so review that before you make any decisions about how to proceed.
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