Navient STUDENT LOAN Wilkes-Barre Pennsylvania

I always pay my bills on time and never have missed a student loan payment in 15 years.  My student loans were sold to Navient and have had nothing but a head ache and hours of my life lost on phones.  I made my last payment to ACS (last company that owned my student loans) in July and have bank statement proving it. In August I made Navient a payment per statement I received. Well then in September I'm getting over due notices and they are stating I'm over due and owe for July and for late fees. I have spent hours and hours on the phone, faxed them bank statements showing checks were cashed for student loans in July and in August and should be up to date. Now I don't want to make a payment because they are going to apply it to a payment I do not owe and for late fees I do not deserve. I speak to people and they promised they will fix it and send me a correct statement and they never do. I spoke to a manager and they told me I have to pay them for July also and it was tough.

I want a corrrect bill before I pay so I know my payment is not going for a payment already paid or late fees I do not owe.