Complaint sub-issue: Trouble with how payments are being handled

XX/XX/XXXX - I agreed to an interest only payment plan with Sallie Mae. This agreement consisted of 6 months of payments towards interest accrued only. On this date I paid {$600.00} during the phone call from my debit card to pay XXXX and XXXX interest payments, {$300.00} for each month. I was told the {$300.00} was the total accrued interest from both of my loans. 
XX/XX/XXXX - I called Sallie Mae to make my payment. I was told I was not enrolled in the interest only payment plan. A {$300.00} payment was eventually taken and applied to my account. The associate told me my card info could not be taken but my banking account information was what was needed to be used. I asked to have these payments be posted on my statement for each month and was told it would be in the next few business days. 
XX/XX/XXXX - I called Sallie Mae to make my payment and was once again told I was not enrolled in the payment plan I had agreed to in XXXX. I expressed my concern that this issue has become a monthly struggle. The associate told me it would be pushed to processing. I gave permission to have {$300.00} be withdrawn from my account for payment then was quickly hung up on without opportunity to ask any questions or have any further explanation. 
XX/XX/XXXX - Still no payment was withdrawn from my banking account. I called Sallie Mae to resolve the issue and they told me I still was not enrolled but saw a note on my account that I had tried to enroll. I was then informed that I would be charged a late fine. I then disputed the fine and then was told my request for the interest only payment plan would be expedited but would not be processed yet. 
XX/XX/XXXX - Called Sallie Mae with my cosigner to make payment and the automated services said they were closed. We were told by voicemail with customer care the office would be open until XXXX XXXX. We called at XXXX. My cosigner sent an email to customer advocate and was notified we would be contacted within 2 business days. 
XX/XX/XXXX- Sallie Mae called my cosigner and he requested to take his issue to escalations which is when he was transferred to the same associate I spoke to on XX/XX/XXXX. Cosigner discussed issues where he was then transferred to processing as I was still yet to be enrolled and was still being held accountable for late fees and " past due payments ''. He was told it would be posted to my account within 24-48 hours. 
XX/XX/XXXX - I sent an email to customer assistance requesting all documentation of enrollment and all notes on my account. Never received a response. 
XX/XX/XXXX - No payments were taken out of my account still. Cosigner received calls from Sallie Mae which then he returned and was placed on hold multiple times as the associates continuously transferred him to various other people and was put on hold for 10 min, heard the ringing dial tone and no one picked up - he called again and was paced on hold again for 5 minuets until he was eventually hung up on. He called back and once again tried to make payment and discuss account details where he was then hung up on for the third time. 

Over the past 3 months I have received emails with account warnings about past due payments and repayment options available- including the interest only payment pan which I was supposed to already be enrolled in. Within my account my XXXX payment was only posted to one of my loans instead of being split between the two resulting in more late fees. I added up the accrued interest for each month for both loans and nothing amounts to {$300.00}. The most that was owed was {$290.00} in interest for XXXX and the other months were various amounts due with less value. I am not sure how the {$300.00} was truly calculated and have been getting charged that amount with no avail to any explanation and with additional unexplained late fees being tacked onto my account.