Over Charged by Xfinity

I moved recently. I called Xfinity in August and said I would be moving, they said to keep the modem until i've moved and then re-establish service. I came into the Xfinity store on 223rd in Gresham Oregon on Sept. 28 and exchanged my modem for a new one, and to turn on my internet. I signed up for $50 per month internet only. On Sept. 23rd Xfinty charged my account $124 (!) Today when I returned the store they said that I had begun using the internet on Sept. 15th. This is Wrong. They also said that I had a cable box at my residence and a movie package that I was streaming. Wrong! They offered a $30 refund today. There is a lot about today that doesn't make sense and I would like an investigation and report as to what exactly was streamed on my account...