Can a auto repair shop hold my car for pmt if they didn't disclose the type and amt of the diagnostic test?

I had my car towed to a repair shop in Chicago (87th Cottage Grove) by AAA around 1/13/2010. The car had sat a week in the 1st heavy snow of the year. After the car was taken off of the tow truck, I put the key in the ignition, and it started. I put my keys along with a detailed letter about what was wrong with the car, in the night drop box. I had called the shop that day and talked to the desk clerk, who instructed me where to leave the keys. I decided to leave a detailed letter explaining what was wrong. I kept a copy of the letter. I had used the repair shop before for an oil change, so they had my contact information, in addition to my contact info on the letter. They have a sign in the window saying a diagnostic test costs $39.99. I called for 3 days, asking when the car would be checked out. They finally checked the car out and told me that the problem was the motor. They told me another motor would cost $1,800. Or they said I would have to pay $100.00 for the diagnostic test, which is $60.00 more than the sign says in the window. I never signed anything, and now they say I have to pay storage charges, in addition to the $100.00 for the test. I'm willing to pay the $39.99 price for the test as advertised in the window, so AAA can tow my car to my home. What can I do?