I have student loans with Sallie Mae

I have student loans with Sallie Mae. At the beginning of the year I requested deferment and was granted deferment, with my new payments beginning May 2013. A few months later they sent me a bill. I contacted them as soon as I received the bill and asked why I was being billed before May 2013. I was informed the same day I was granted deferment it was cancelled. I let them know I did not cancel the deferment request and would like for them to fix whatever error cancelled it. Their response was to lower my payment by $20. I have spend months now try to get them to honor the deferment letter they sent to me. I have asked for them to only contact me via email or mail so I can have written proof of their responses which they have not done.