Can't Return Unused Equipment

I have been attempting to return unused equipment (a replacement modem I was sent unsolicited that I didn't need and cable boxes since I have unsubscribed). This has been going on well over a year. Whenever I try to get a return label, I fill out the information and get an error saying "Sorry, this isn't working right now." I've tried over multiple months and devices so it is an Xfinity error and not a computer issue. I have reached out to support with no advice. I just got off a live chat where they told me to call UPS, and I said I'm pretty sure I need a label indicating what it is and where it should go...but the agent logged off. I don't have time for a road trip to a store and just want to get rid of this stuff I don't need and keep getting charged for.