Product: Student loan

Several years ago, I was paying off a large student loan debt with Sallie Mae via online payments. Around the time that Sallie Mae was moving the servicing of its existing loans to Navient, they removed my account from online access so I could no longer view the balances or make online repayments- for no apparent reason. This made it more difficult to make and track payments and marked the beginning of endless Navient problems. I attempted to rectify this on multiple occasions and reinstate online access but was never reinstated despite numerous calls. 
I had previously made some extra payments by phone to Sallie Mae to pay certain loans in full and close them out early. There were never any issues with that process, so I made a large phone payment to Navient to close several loans around XX/XX/2013. I had given specific instructions over the phones and received verbal confirmation that payments were made to specific loans to pay the balances in full and close these out. Almost a year later, I was shocked to receive an overdue balance notice in the mail for some of the loans I had paid off. I called Navient customer service at least a dozen times requesting an accounting of payments made and how monies I paid were applied, and spoke with numerous reps and supervisors in the call center with no action taken to provide me with an accounting. I was told that despite having an open overdue balance, Navient had supposedly sent me a refund check for over 300 dollars. They had no viable explanation for why I would receive a refund if I still had outstanding debt. Needless to say I had no record of receiving this check, and would certainly remember it had it actually arrived since it clearly indicates an error. When I asked about the credit, they attributed it to a difference in interest that had been quoted at the time of my payment but had not yet accrued because it was done mid payment cycle and therefore needed to be credited. I then clearly illustrated to the rep that this explanation is mathematically impossible because the dollar amount of the credit was equal to roughly a year of accrued interest based on the loan principal amount times the interest rate. He agreed, and stated he would look into the account further but took no action, again, to provide the detail on the account. I finally asked for a formal complaint process and was told the only option is to write a physical letter to an ombudsman and mail it. I did just that and wrote a detailed letter, mailed it to the address the call center provided and waited approximately 3 more weeks. When I called to check on status, I was told my letter was never received. Since no accurate balance or payment history was made available to me and errors with misapplied payments were never corrected, I did not make any additional payments after this time since I did not know what the actual balance was, or which loans were open that ought to have been cleared. Quoting an incorrect balance or loan numbers did not stop Navient from their collection efforts against me and from ruining my credit despite failing to address and correct the issue. Aside from endless calls for collection, they reported and continue to report this as bad debt each month for multiple years. They have added interest and penalties to the balance and have validated this debt as accurate when I've disputed it with the credit reporting agencies.