Billed after service ended and account was cleared

I am receiving messages from Xfinity threatening to place my "past due balance to avoid my account being placed with a collection agency." 

I canceled my Xfinity service and returned my router 2 months ago when I moved to a place where I have internet already provided. At THAT time I was billed the regular $73 service bill for the period past my move, but I explained the situation to an employee at the Xfinity store when I returned my router, and she was able to clear the charges and end my services and autopay function.

Now, two months later, I receive an $88 charge for a service I haven't had or used in 2 months! I spent 30 minutes on the phone with customer service this weekend, who assured me the bill would be cleared and gave me a confirmation number. This morning I receive a text from Xfinity threatening to send the bill to collections. I am going crazy.

There is NO direct message feature on this account; the only icons at the upper right are my profile and the notifications bell icon. Please explain BETTER how to contact Xfinity directly in writing. I am sick of calling and being passed from person to person only for this problem to keep cropping back up with MORE money I supposedly owe.