Sounds like a nightmare, but - if you payed by credit card, it doesn't have to be.
If you did use a credit card:
First, try and get them to fix the idle, get a receipt or some proof that you gave them another try.
Get an expert opinion - estimate from a dealer to REALLY fix the problem. Dispute this amount of the charge from the old repair with the credit card company. If the new estimate is higher than what you paid already, dispute the whole thing. They'll side with you and the shop will be out the entire amount.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino