Well I ran down your list and ball-parked the parts prices. Minus the O2 sensor which I do not know the price of, I came up with approx. $200. I suspect the O2 sensor was about that price as well(or more). That leaves about $200 for labor.
I think you did ok as far as price goes.
Not sure I follow 3 qts of gear oil for a 260 diff. My 126 hold about 1.2 qts. Your engine holds 6.3 qts. and they rightfully charged for 7.
If your brake fluid is cleaner looking, then they likely changed it.
The only way you're ever going to know with absolute certainty that anything gets done is to simply do it yourself.
Many of the tasks just performed on your car are relatively simple diyer tasks. Lots of info here in the search facility that could help you do-it-yourself next time.