So much to comment on here, some things I will agree with, others not so readily depending on the scenario and circumstances.
Check-outs are normal, but some clubs make it a profitable venture, such as when I did my complex endorsement, even though the instructor said everything was good about 5-6 hours in, at that point I asked for the endorsement and he said the company requires 10 hours. I did feel taken advantage of, but I used this to get some IFR long xc in as well.
Did they tell you up front that they required 10 hours for the checkout?
Also, you/they need to differentiate between the complex endorsement and the aircraft rental checkout. They're not the same thing. If you were ready for the endorsement in 5-6 hours, the CFI should have signed you off for that but not the aircraft checkout if it requires 10 hours. Not a whole lot of practical difference perhaps, but maybe you want the complex endorsement for some other reason than to fly that actual airplane.
Another point is the instructor added 0.2-0.4 hours of ground time to fill out the logbooks and the debriefing post flight.
I don't see an inherent problem with this. Did the CFI not spend at least .2 to .4 with you on the ground? Are you expecting the CFI to work for free during those times? I charge "handshake-to-handshake" time. If we're together for 1.8, I charge 1.8, regardless of whether the flight was 1.5, 1.2, whatever. My time is in no way connected to the airplane time. Now, you get value during that time, but sure, some of it is spent just getting in the airplane or closing the hangar door, etc. Or even the minute that is spent writing in the logbook.
Another place I went to, I called up to book the plane and instructor, advised of weight and balance ahead of time. When we arrived, the plane was at the back of a box hangar and needed fuel to be removed to meet W&B.
That's a tricky one - I've had that situation before, where the school/club/FBO policy is to fill it up (or to some other level) after every flight but you need something else for yours. If you're talking to the person at the desk, chances are good that word will not get passed down reliably. About the best way is to take control yourself and call the previous renter and tell them (if you self-fuel) - or call the FBO close to the time you see them land, or be there in person to watch the fueling. That really is one of the big hassles of renting. Many owners I know with higher-performance airplanes fill up BEFORE a flight for that exact reason.
At the end of the flight, with no ground training at all, they added 1.5 hours ground time in addition to the flight. I never returned and have told everyone about their overcharging practices as a heads up.
Obviously you shouldn't be paying for time that didn't happen. But the time to correct that is right then when you pay the bill.
I’ve joined a club that doesn’t charge ground time
I do know some CFIs that don't "charge" for ground time. Well, sure they really do, they're just rolling that into their flight time rate. I've never understood why they do it, or why they claim it "makes the math easier", but effectively if they are doing so they're charging a higher rate for just flight time than they would if they charged for all time. But if that's what they want to do, no problem with me. If they charge $60/hour, a 1.5 hour flight bills at $90. If they actually spent 2 hours with you, they are effectively charging $45/hour. If that rate is okay with them, then it's none of my business.