How to get rid of this Flex box I don't want

Xfinity shipped me a "free" Flex box that I don't want and won't be using. I was going to throw it up for free on Craigslist but decided to do some research first. The internet told me I must store it somewhere for a year and then return it to Xfinity otherwise I get charged $120. The box had a card in it with return instructions, but when I went to xfinity.com/returns it didn't show any devices that I could return so I couldn't print a shipping label. Also someone said it might mess up my internet plan if I return it early.

How can I get rid of this unwanted box?