Plan not updated and keep getting billed old amount.

Hi, few weeks ago, I corresponded xfinity through call to get my plan changed from $70/month to $50/month (-$10 discount with link to bank account). But I am noticing that the billing on my account has not changed. I have corresponded with xfinity via chat agent two different times, twice confirming that the charge will be $40/month despite what is on my account. But come the billing date, I was charged $70. Please fix the issue. If I can just correspond by email, that is easiest, but it seems that I can't, and I keep on getting this run-around during weekdays where I am busy with work, and it's super annoying. Thanks.