Charged for Service After Canceling

I asked xfinity to cancel my service on the first day after my contract expired.  I asked them to confirm I wouldn't be charged an early termination fee since I was canceling for a date after my contract expired.  They confirmed no early termination fee would be assessed.

I then paid my bill in full which included all the days up to the cancelation date.  I never miss a bill.  I like my credit score where it is.

A month later, I check in on my account to make sure I don't have an outstanding balance.  Sure enough, they charged me for service for days after the requested cancelation date.  And they charged me a $10 early termination fee on top of it.  Where's the logic in that, right?

I call and explain the situation.  The customer service representative is very polite.  After reviewing notes and documents, says she will wave the early termination fee and credit my account accordingly to zero out my account, then apologizes and promises to make it right.

Tonight, two weeks later, I check my balance to make sure everything is processed.  Nothings changed.  It still says I owe the same amount.

Fearing xfinity will end up messing with my credit score, I paid the bill tonight.  But this is crazy - even when they say they will take care of it, they won't.  Not sure what to do.  At this point, I don't think they care.

Good luck dealing with them.  I guess they can just charge whatever they want - even when you are no longer a customer.