IC 24-4.5-3-203.5 does provide for a late charge of what is $16 as of 7/1/02 on any payment not made in full within 10 days of the due date as long as your contract provides for it. This is for all accounts open end, closed end, etc. First mortgage loans are not under this provision and can be in whatever amount as long as it is properly contracted for and disclosed.
You can charge a late charge of $16 regardless of the payment amount. We charge 5% of the unpaid payment or $16 whichever is less with no minimum.
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The opinions expressed are mine and they are not to be taken as legal advice.