Can we charge a late charge on a loan past the maturity date that has an existing balance?

We charge a late charge percentage based on the full amount that would have been due and payable at maturity. I would check the wording in your Note regarding late charges

We charge a 5% penalty on the payment amount due if the payment is 10 days past due. So, if the loan matures and we have not received payment, they would be charged a 5% late fee on the unpaid principal balance, up to $999.