Charge for Flex Box not returned?

We got a new WiFi router a few years back and they sent us a Flex box too. We never ended up using the Flex box and packed it up somewhere when we moved 2 years ago and can’t find it now. We just cancelled our Xfinity service and they’re saying we need to return the Flex box. Will we be charged if the box is not returned? Why was it not specified on the package it needed to be returned like it stated on our WiFi?