Verizon

In the middle of September 2015, I had a need to bump up my data amount from 10 to 15 mbs. On the website it stated this would add $20.00 per month to my bill. A few days later I was looking at my account information on Verizon web page and saw that my new billing amount would be approximately $110.00 more than normal. I immediately called an account rep and requested everything returned back to the way it was. I was told that it had been taken care of but the next bill showed the higher amount. I called them again and was told it was because of how they prorated changes to the account. I tried to relay to her that there actually was no change to the account but couldn't get her to remove the extra charges. I continue to have the extra amount on my bill but refuse to pay through threats to cut off our service. How do you make your point with this company?