My car has been acting strangely since yesterday. Its idle went down, the engine vibrations became stronger, and the car easily stalls. I went to a mechanic a while ago and he said that he would be changing the piston rings and the cylinder gasket head. One piston ring set cost me 60 bucks while the gasket head cost me 16 dollars. Of course, the mechanic will be the one to install all of those. What do you think would be fair to give to the mechanic after he has repaired my car? What should I give him if the problem is not solved?