Sallie Mae Student Loan Complaint

Sallie Mae Student loan department,

Non-federal student loan Dealing with my lender or servicer Trouble with how payments are handled District of Columbia

I signed up for auto payment on my Sallie Mae account, to take advantage of the reduced interest. Because Sallie Mae does not allow me to set up my own auto payment amount, the $25.00 dollar payment is not enough to cover the interest. So I have tried to ways to make additional payments to pay down the principal. One way is through the Sallie Mae site, but my payment was used for interest. When I called Sallie Mae, they said that it was because I paid before the due date that I should use my banks Bill pay. I have now used my bank bill pay, and my payment was still used for interest. I called again and Sallie Mae stated that it is because it was a day after my due date. I have auto payment so I do n't understand how my additional payment would be used for future interest.