Charged $100 for technical fee which i didn’t ask for

I’m new to Xfinity and I am regretting my decision. I went and picked the Modem up myself to avoid extra costs, I am a student new to the city. When i got home i connected the modem and it worked for only a few hours. I had a lot of work which was to be done online. I called them a technician was sent out after two days. He said the modem they gave me was faulty and replaced it with a new one. My next bill came and wow they charged me $100. Is this some sort of scam to get money? you give faulty modems and force people to pay extra? $100 for something that WAS NOT my fault. I want my money back but the system is so great you can’t possibly contact anyone.