Customer service representatives not being held liable for what they say about the services

In November I contacted customer service via the chat option, the reason was that my bill had increased from $44.95 to $49.95. I was told that in September 2014 when I upgraded my phone service to unlimited it was supposed to be $49.95 + taxes and was told that the bill was going to be 49.95 total until 07/20/2015 and I missunderstod what was said in September. Unfortunately I didn't have any records of the conversation in September so I could not verify it but I did take screenshots of the November conversation just so I could verify what had been said to me.

Now my bill has been changed again to $64.87 and I am being told that what the representative told me in November is incorrect.

It seems that AT&T will increase their customers billing at AT&Ts convenience using the argument that the representative just told you the wrong information or you the customer missunderstod what was said.

AT&T say the promotion is no longer valid so we can not fix it and now we will increase your bill and there is nothing you can do about.

How do you run a company and not hold your representatives liable for the statements they make.

Fortunately I have the screenshot of the conversation on 11/11/2014 with the representative stating that my bill was supposed to be $49.95 until 07/20/2015.

If any other customers read this let me know if AT&T have done the same thing to you.

I hope other customers read this and start using the chat option so you can take screenshots of your conversations or record your phone calls with AT&T that way you have proof of exactly what has been said and as a customer you can feel confident when you talk to AT&T who try to blame us as customers that we just missunderstod or that the previous representative was incorrect.

How are we supposed  to feel confident doing business with AT&T when AT&T do not hold their representatives liable for their statements.