Having trouble with my bill

I ordered new service with an estimated monthly bill of $50 per month with the email confirmation of $50 per month(Includes new and continued services, Excludes one-time charges) and now my bill is $79 per month. I have my own modem, am a Veteran, and have a budget of $50 dollars per month and would like to remain a customer with Xfinity. I spoke with 6 different xfinity representatives they all kept trying to sell me things I wont use. I only need internet at least 600-1000mbps for $50 per month.