I called Navient at the beginning

I am a grad student who takes four classes a year. During the summer, I received notice that my loans were deferred and the only payment I needed to make was the interest, if any. I called Navient at the beginning of the fall semester to ask if that was the only payment I had to make. The man said yes. I continued to pay $56.32 for September, October and November. After November’s payment, I started getting daily phone calls from Sallie Mae saying I was late on my payment. I then received a written notice that I was late. I have NEVER been late on a payment and I called to make sure I wasn’t. They now say I owe almost $700 and that my account is over 50 days late. My problem is that they needed to send me a notice via mail and called me in September if that was the case, instead of waiting until November. I am an excellent customer with an over 800 credit score. I would have made sure that I made the payment. I talked to several customer service people, including a supervisor, and wrote a letter to rectify the situation, but no avail. Sallie Mae made the mistake and now it will adversely affect my credit score when it was clearly their fault.