If the mechanic overcharged for the parts, (say for example, ona’ah), I would think that it would be the mechanic’s obligation to take it out!
I once asked a shaila: If I order something at a certain price and it’s delivered but invoiced at a higher price, can I keep it and pay the lower price?
I was told no, because ????? ???? is wrong on the part of the seller, but unenforceable on the part of the buyer. However, I can tell them that they have to come pick it up; I have no obligation to send it back (but, of course, I can’t use it).
The variable in the case of a mechanic is that within a certain price range, it can be assumed that the buyer would have agreed, because he definitely needs tires. I suppose each case would have to be evaluated individually.