Nissan of McKinney

Well let me start with my mother purchased 3 new vehicles from Nissan of McKinney over the past 10 years and was always happy going back to them. good enough reason for me to try them . Wow I was wrong! we settled on a price and financing, all was great till I logged in to make a payment. Four thousand more than we agreed too. I went in the next day to investigate and found they had added on a 4 year warranty that I never agreed to. And when I talked to the manager he said there wasn't anything he could do. WHAT? I've never been treated like that buying a new car. Well they lost two customers and now im going to figure out my next action....