Xfinity charging me for service after I canceled

Apparently this is a common problem, and I am yet another person getting charged for the month after I canceled.

I'm making this assumption, that others have made, that Xfinity always bills a month before the service that they will give.  In other words, you have to pay for the service, before they will give it to you.

The bill I'm being charged for was due on Oct 16, so my assumption was that this would be for service AFTER Oct 16th.

I canceled my service on Oct 7th, which was well before Oct 16.  I also returned my Flex box to the local Xfinity store, and was told that I would be receiving a refund of $68.42

I just received another bill that does show this credit under Services removed of $71.80, yet I had a Services added on Oct 13th charge of $3.38, which brings us to $68.42

But I'm being charged for a month after, so another $101, on which now I owe Xfinity $32.58

The other confusing thing is that the services that the bill was for, was between Sep 28 - Oct 27, which was definitely not a month after the Oct 16, but right in the middle of the service cycle.

The rep I talked to on the phone, agreed that they bill a month ahead for service, but then said I still owed that $32.58 for a bill due on Oct 16, when I canceled on Oct 7th.

None of this makes any logical sense.  We just went round and round about this.  This is why Xfinity loses customers, I'm so glad I no longer have to use them. 

I want my refund that I'm owed.