I know the gas caps can start deteriorating and throw a code. My point was that if the OP took it in for a cooling issue and nothing else, I can kind of understand the shop pointing all the underhood stuff as they poke around, but going as far as the gas cap seems like padding the bill - unless there was a code. This list really depends on what the OP asked for.
Reminds me of my partners MINI that went in for a O2 sensor covered under warranty and came back with 5k laundry list and basically told him the car was one key turn away from self destruction. Some of it was legit such as bushings and all which I had taken care of by my independent but the rest was a tactic to make service dept numbers go up and \ or send him off with a brand new car. 18 months later it still runs like a champ.