You are correct in that you can not do a change in term. It has to be contracted for, and you are also correct that it is not adjusted for inflation.
We don't contract for this fee, but if I recall, without researching, the fee can be added to the balance, or if it's a statement or revolving loan you would show it on the periodic statement.
BTW, don't confuse this NSF fee with one that can be charged against your borrower for writing a NSF check against a HELOC. That fee is not allowable under the IC. It can only be charged for a NSF payment to the account.
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The opinions expressed are mine and they are not to be taken as legal advice.