Your experience is “Standard Operating procedure” for many companies.
They charge you or over charge you in error.
Most people won’t even realize it and pay.
For those that do, it is a “Customer service” issue.
Since you had cancelled the service you actually realized the charge was erroneous and were willing to fight back. I think there are many folks that would just pay the charge to be done with it.
The fact that it went to a collection company so quickly leads me to think they where up to something a little different.
I had a situation with Comcast many years ago. They were charging me late fees on late fees. I had all my paper bills from the first day I started the service and was able to get a rep on the phone that had access to my entire history.
They wanted to “Start” with the current month and work backwards. I said NO. Lets start at the beginning, since this is the ONLY place where we can agree.
After an hour they ended up owing me two months worth of service.
What totally pissed me off was getting that “collection demand letter” without even getting some sort of invoice from Time Warner.....We’ve got a credit score so high that the “finance guy” at the car dealership we visited not long ago told us he’d never once seen one so stellar..............and I for damn sure wasn’t about to let TW screw with that.