Am I getting charged for the full month even if I use the service for 2 days?

I want piece of mind from my wireless service. That pretty much means, no bill surprises. I had wireless service with AT&T for 9 months or so, and it created many unpleasant surprises. For example, when I added a new line to my account, surprises.  I am a fan through employer, retail employee said your application fee would be waived. I received my first bill, there is not only activation fee but also pro-ration due to the new line. The former is an unpleasant surprise, the latter is fair.

Then, in June, I decide to move to another carrier b/c I can get an unlimited service for pretty much the same price. I remember my bill cycle date as 21st, it turns out it is 18th. Still, I say go ahead, you are not happy with the service, cancel and move over. I proceed, a week later I receive a bill, they say my final bill. It correctly credits back the earlier activation fee to my bill, thank you, but instead of pro-rating my charge from June 19th-June 21st, it charges me from June 19th to July 18th.

Yes, you are reading exactly right, I am paying the exact same bill as if I had not canceled. By the way, today is June 28th, we are not even in July yet and I am no longer using the service. Still according to this bill, I need to pay for the service until July 18th, it is irrelevant whether I actually use it or not.

This makes me think maybe AT&T people are not smart enough to figure out proration. Then I remember, they were able to accurately and correctly pro-rate the new line I added a while ago.

Then, I think, maybe if I call the customer service they would help. I talk to an agent, we cannot get on the same page. I ask for a supervisor, she tries to explain to me how AT&T has the right to charge for the full month when I used the service 2-days, but I can hear her voice is trembling, maybe because she feels like **bleep** trying to defend a non-sensical policy.

Then, she refers to "an internal" document, which allegedly says AT&T does not pro-rate monthly bills no matter when you cancel. I am asking if I can see that document, she says it is internal but maybe if I can go to a retail store they might be able to print one for me. We all know, I am not going to do that.

So it does not look like that no proration statement is on my bill, and maybe not even on the T&S I signed while establishing an account. But, they leave me no choice but to believe that they have a document, and they are absolutely incapable of doing anything to help me.

Now I am trying to interpret this whole experience with a positive attitude, but really cannot stop feeling like duche who got ripped by  a company in a sneaky way.

I am head of a household that makes six figures a year, so we have the financial power to pay the ~$50 bill. But if I pay that bill, that will be *the* final bill I pay to AT&T, as I have no interest in doing business with companies who are ripping me off.

[Edited due to word filter avoidance.]