Unwanted service added without consent, lying to customer

I visited the xfinity shop at 2825 El Camino Real #100, Palo Alto, CA 94306 today for an Internet plan, and a male staff member helped me. I mentioned I want the 300mbps plan with promotion ($40/m) while he said I would have to rent a modem for $15 /m. When I got back home, I noticed there was a fee called now tv-Latino added to my bill which was exactly $15 /m. It took me 1h to figure things out and almost wasted half of my day. Plus, the staff member picked up several phone calls and texted messages during the service , leaving me aside. 

this kind of behavior is extremely unacceptable and dishonest, if I had not figured that out , I would have had paid for extra fees every month until I surprised myself with the bill one day. Your salesperson is a liar , the worst shopping experience ever.