If the invoice was paid too late, you will see the applicable interest amount here. You may also choose not to apply the interest to a certain invoice. If so, you can no longer print an interest invoice for that invoice.
In order to calculate interest, you must enter a late payment fee for that customer in the customer register. You must also enter a general percentage for the late payment fee under the section Interest invoices under the Sales tab in the Settings procedure.
You can also use customer-specific percentages, these percentages must then be entered under the heading Exceptions under the Info tab in the Update Customer procedure. The customer-specific percentage (other than 0.00%) overrides the general percentage. The company currency is shown next to the interest amount.