Billed after closing account

I cancelled my xfinity service and closed my account entirely in-person in an xfinity store back in July. When I cancelled I was told that was it and that I didn't owe anything. I returned my equipment at the same time. I got a letter in September stating that I have an outstanding bill that will be sent to collections if I don't pay it. No details of the charge are available anywhere. Because I closed my account, I can't log in to see what the service period I'm being charged for is. The bot can't do anything other than repetitively ask me to pay my bill.