Interest charged on my Edgars "interest-free" account!! Horrible service!

My Edgars installment keeps increasing for no appearantly reason regardless of the fact that I pay in advance on other months!!! Also I have interest charged on an interest free account?!! How??? I tried calling them numerous times and I run out of airtime waiting to be put through!! I have been with Edgars for years, I'm so annoyed and disappointed at this horrible service and shady increases and interest!!