I appreciate the follow-up posting.
A civil suit is possible to address inaccurate late fees and penalties being applied by a loan administrator. However, that is typically a last resort as the time and expenses involved may outweigh the cost of the late fees and penalties. I see you have written four letters, but I am unsure of the approach taken otherwise.
A succesful approach is to take up such issues in writing with a loan administrator in a clear and concise manner. A chronological accounting of events including marked exhibits such as notices, statements, and proof of payment is the first portion of a demand letter typically used in such scenarios. Such a letter would then typically include a demand to correct account errors and cease collection attempts. It is generally a good idea to identify responsible management at more than one level of the entity and to send a copy to both via certified mail, return receipt requested. This may seem like a lot of work, but it is much less work than preparing for a civil suit should informal resolution not be succesful.
If the written effort is not succesful, then consider attempting to utilize the US Department of Eduction Ombudsman which can assist in such matters. Here is the site for that office, at:
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Thank you so much