I made an appointment for replacing a valve cover ...

I made an appointment for replacing a valve cover gasket, spark plugs and air filter in the first place, but the day before I had some overheating issues. Therefore, I asked them to fix this issue first, and I told them to double check the thermostat first and to give me some updates before they do anything.
After a couple hours, still NOTHING! I went back to the Service Advisor and ask for an update, and the mechanic had NO CLUE! My car was apart, and he didn't know what was the issue except he mentioned a potential head gasket issue which could cost $3000-$4000!!!!!. I didn't buy it so I asked him again to check the thermostat!
After a while, the Service Advisor came back to me saying that they are not sure yet but it may be the head gasket so they wanted to keep the car for additional and it it is, it would cost $1500-$2000!!! Yeah, right...
They also tried to sell me a new car, and scare me if I take back the car, but I DID take my car back!.
I went to another place that found out in 10 min that it was a defective thermostat!
Problem fixed for less that $100 !!!
I can't believe a dealership tried to RIP me OFF like that!!!
DON'T EVER GO THERE!