Neurological Specialist PC

I used this doctor for a sleep study. I did not have insurance, so I called them to find out the costs. I agreed to pay up front in cash, which I did before I did the sleep study. Their office called me and sent letters to me before the sleep study making sure that I understood that I had to pay when I arrived for the sleep study. I assured them that I would have the money when I got there. I got to the hospital, paid and had the sleep study. All was fine. The staff was wonderful and the night passed by very nice. Within a few days, I went back to the doctor for a follow-up. When I left I asked if I owed any more money and I was told that I didn't. However for several months they have been sending me a bill for nearly $500. I don't see how they could bill me for something that I wasn't told about up front or at least warned that there could possibly be other charges after my visit.