I have always thought an appropriate gift is a six pack of ice cold beer for a small shop or a case for a large shop.
Another possibility is a gift card or certificate to the Snap On truck. A princely amount for a mechanic who saved you a major expense. If you were miffed at the mechanic, you could make the amount <$20 as nothing on the Snap On truck costs that little forcing the misfortunate to dig into his own pocket for the balance.