Photographer question?

I am having a photographer come to our house and she told me on the phone months ago that she charges $100. When I called her a month ago to arrange the date she said her price is $150. I agreed and asked if I will be charged more if we go over her hour session. She said she wouldn't charge if it goes over but that one time she was at a person's house for 4 hours and she didn't want that. I assured that would not be the case. I confirmed with her the time and again asked if I would get charged if we went over the hour, to be certain I wouldn't . Now she tells me if you go over her hour she charges in increments of 40 dollars! How should I deal with this?