LOL!
That would be a good idea.
Those guys definitely sound like professional idiots who know more about getting money from unsuspecting innocent people like yourself than actually being honest and fixing the problem correctly. That really ticks me off!
But props to you for replacing that in cabin filter! I don't think I've even done that yet.
I'm not actually a mechanic, but I did start out kind of like you did, sort of figuring things out on my own and getting pissed at mechanics who didn't know what the *%#$ they were talking about. Now that you've seen the light, hopefully you can find a more reliable mechanic or even take care of some of the more simple maintenance tasks. Speaking of oil changes, I taught my little sister how to do that a few months back, and she actually enjoyed the experience, which was her first time actually getting her hands dirty on her own car. It's amazing how little do-it-yourself things can not only become fun and rewarding to do on your own, but also saving lots of $$ in the long run!