As far as the cruise control parts, if you want to give that a shot on your own with your cheap parts, go ahead. If they don’t work out for you or last very long, it’s your problem. Ditto for if the repair doesn’t go exactly as planned, they send you the wrong parts, etc. Repair shops have a tendency to use premium parts from a known good supplier because they want their customers to be satisfied with the repair for a good long time. They will also mark them up, not to be shady, but to stay in business. Markup is not a rip-off tactic, but is standard in any business with an overhead, especially an overhead as substantial as an auto repair shop, which is way, way more than most people would care to guess. They also know that if you have a good experience with getting your car repaired by them, you might tell someone you know, but if you have a bad experience, you will tell everybody you know and everybody you see who will listen (and probably some people who don’t care to listen). Before attempting anything, though, it may not hurt to consider cigroller’s comment about “if you have to ask…” That was actually the first thing that I thought when I saw the title of this thread.
As far as the a/c repairs, they are not selling you unnecessary parts “that seem to be working just fine”. Any time any part of the air conditioning system has to be replaced, the receiver dryer and orifice tube (or expansion valve, depending on the design of the system) have to be replaced. The orifice tube/expansion valve often get clogged with contaminants introduced to the system by the failed parts, and if a leak is present and during repairs, the desiccant in the receiver dryer will become contaminated. Replacing these parts is standard procedure during any a/c repair. If you want it done otherwise, a good shop will refuse to do it. A not-so-good shop will do it, but refuse to warranty parts or labor because they know the repair is not being done properly and will either not work properly or fail very soon, putting you back at square one. Doing the repairs yourself is not a viable option unless you have a background in working with refrigeration equipment and have things like a deep vacuum pump, manifold gauges, and a highly accurate scale. If you don’t have those items, which are absolutely necessary for a satisfactory a/c repair, buying them will set you back several hundred dollars if you buy cheap stuff, closer to a grand or more for better stuff.