Lost Free Xfinity Flex Box which was never told to return and now they charge you $100+

I am moving and need to cancel the Xfinity plan, they require you to return the Flex box which was free of charge at the beginning and did not tell me to return. They charge you $100+ for something they did not specifically say in the beginning, I did not want the flex box in the first place but they give you anyway.

This sounds quite absurd to me and sounds like a scam. This kind of customer service really is "one of a kind", I've never experienced this with any other internet providers.