I did my own roof, and learned a strange thing about getting price quotes.
When I would call the roofing supply store and gave myself up as a do-it yourselfer(by my lack of knowledge on subject) , they seemed to add about 30% to the materials quote.
I learned this by accident, as I called the store, and described, using general terms what I was looking for, and got a price quote. They gave me item number, etc. When I called back a couple of days later, I was in a hurry, and must have sounded like a contractor ("need 14 square of GAF shingle item #XXXXXX delivered rooftop to house at location") and he gave me the price that was about 30% less. From then on, for all materials for the rest of the job called the roof store (of the same owner) farther from my house, talked to the guy, got item numbers, etc. Then I would drive to the closer store, walk in in my roofing clothes, rattle off the item #'s and amounts, and they would give me a cheaper price.
It didn't seem all that fair to stiff the little guy, but it is their business and they can charge what they want. I was just glad I caught on, and used thier own unwritten rules to my advantage.
BTW, 3 layer tear-off, roof was 14 square, a couple of valleys, 2 story, read a lot before I started, got my brothers to help, took a week, saved probably 2 grand. Still look up with pride, had 2 winters of icedams, and it is holding strong. My proudest household achievement.