Transfer from postpaid to prepaid and billed for extra data!

I have just transferred from postpaid to prepaid plan. But I still have to pay for one more postpaid bills including 3GB of data in next billing cycle( Nov 24 - Dec 23). I know that I cancelled my postpaid plan days after new billing cycle began and I am willing to pay for that. But am I supposed to enjoy the data I paid for ?! I didn't change my courier so it's perfectly ok for At&t to continue to serve me these data! I was also not notified about the rule of "billing one month in advance" when I first signed the contract! I have called several customer service numbers and they either told to call another number or couldn't be reached. At&t and its service is so disappointing!

Solved: After calling customer service number after customer service number, finally found the number that can help me. (Why can't At&t post all necessary phone numbers online instead of asking customers to call again and again?) And after ten more minutes of waiting, I was told by a staff who finally answered my call that at&t will give me credit to cancel my extra bill. If I had not noticed the new bill, the auto pay will cost me $80 for a service that I couldn't enjoy. If I wasn't so persistent in calling, I might give up and pay the bill already. Life is really full of traps.