I don't have any good advice other than to take it back to your mechanic and complain (as you seem to have done). My dealership tried to charge me for a mistake they made. I refused to pay. They then said, "OK, we will only charge you $90 for the diagnosis." I refused to pay that as well. They caved and I didn't pay a dime. I never went back there again.
I found what seems to be a good neighborhood mechanic where I live using Click and Clack's website:
http://www.cartalk.com. They have a system to review mechanics.