I believe this happend to me with gynocologist a few yrs. back. The next time I went for my yearly pap test & he asked me how I'm doing, I told him I could not tell him without them billing me extra. Gyno doc was the one originally discovered my thyroid problem. He still checks my thyroid, but I don't get a double billing. I don't think all the dr.'s actually know what happens with the billing & you have to be challenge the bills at times.
I actually switched GP doc as you could only talk about 1 issue per visit. I had been complaining about my neck for 4-5 yrs. & he never did any testing for thyroid.