Xfinity threatening to send my account to collections, but won't let me pay the bill

I cancelled my service last month. I received an email today saying I still owe $63.99 and they are threatening to send me to collections. I went to the website to try and pay the bill, but my account no longer shows up and there is no option to pay. I tried the app and had the same issue.

I tried support and was forced to use the automated chat (as always). I told it my issue, and it said it would put me with a live agent. It then gives me the classic error saying it can't get me with an agent and to schedule a callback. It then fails to schedule a callback.

Next, I called them. I had to deal with yet another automated system. I give it my zip code to look up my account, and it says there's no account associated (as expected, since the account was closed). I tell it to connect me with an agent, and it says it can't do that until I provide a valid account number. It's an endless cycle.

I just want to be done with Xfinity. How do I escape this hell?