Xfinity "told" me I could upgrade my TV boxes to get 4K video. But apparently it will not work without Xfinity Internet.

I could not find any information on what model of box Xfinity was sending me, so I had to order it to find out. After it arrived today, I was able to google "XG1v4-A" as it was printed on the back of the device. Turns out you need to have X1 internet to be compatible with the device. I dumped Comcast internet a few months back for something MUCH better. I guess Xfinity just shipped it without any thought to whether the customer could actually use it or not.  

So how do I return this incompatible device (unused obviously) in the best possible method, so it won't get lost in the Comcast systems and I get charged for 2 boxes?