My ISP bills very strangely.

Dates of invoice:

May 14
June 14
July 17
August 19
September 20
October 23
December 01

Since I signed up with them, rate has fallen by $5 and when I called, they told me nothing can be done because I was locked in the previous rate. Furthermore, my current rate was supposed to be for the first 6 months only with a $10/month increase for the last 6 months. So far, I'm getting half a month free and the rate hasn't gone up at all.