Att live chat representative make fake promises

I just move to a new community and it seemed that ATT is only internet provider in the area, so I have to go with ATT. At first, everything is all right, the installation was quick and the signal is pretty stable. One day I was chatting with a representative through online chat, the representative(Katie) offers me a special 12 month promotional offer stating that she would offer me a triple speed (18mpbs) with the price unchanged. I double, even triple checked with the representatives that the price will remain unchanges while the speed tripled, after I got a positive answer, I decided to upgrade.

But when the bill came I realize the price actully increase about 10 dollars more than my original price. I freak out and contact them through att live chat, again. The manager asked me to provide the eviddence, which is screenshot I took, I tried several time to send the screen shot to him but it just won't go through, I asked whether he can provide me with a email, the manager won't provide me one. All then can do to downgrade the plan and won't accept the offer that the representative offer me. 

If they can't keep their promises, why they offer that promotional offer to me?? I am so disappoted to ATT now, if there's another sevice provider, I would defintely swith.