I can show you how to enter interest charges into customer invoices.
You'll want to either create a separate invoice for the interest or add another line item on the past due invoices. Let me show you how to set up the interest item:
Go to the Sales menu and select the Products and services.
Select New and click Service.
Give the fee a name something like Interest Fee/ Charge.
If you have a fixed interest rate, you can enter it if none, you can leave it blank.
Fill out the rest of the fields as needed and ensure to select the correct accounts to track this item.
Click Save and close.
Now, here's how you can add the item on the new invoice:
Click the +New icon, then select Invoice.
Under the PRODUCT/SERVICE column, add the interest item.
Enter the amount manually in the AMOUNT tab.
Click Save and close.
If you want to add it to an existing invoice, just open the overdue transaction to edit and add the interest. Review the total amount before saving it.
You can check out these links that will help customise your sales forms: