Xfinity is charging me for their mistake

Xfinity has terrible service due to their [Edited: "Language"] modems. To make it right, they sent out an agent who indicateed that they were having glitches with my current modem. He set me up with the modem that is one model prior. THEN TODAY I GOT AN EXTRA $100 CHARGE ON MY BILL FOR THE "INSTALLATION FEE." No one even told me I was going to be charged. This is UNACCEPTABLE