You can notify customers of your policy regarding late-payment interest directly on your invoices. Make the change under My account -> Invoicing settings.
However, if your invoice is overdue and you wish to charge statutory interest or recoup other late-payment compensation, there is no specific field to add this to your invoices as of yet.
Adding interest on an overdue invoice
1. Go to Invoices and select the invoice.
2. Click to Modify the invoice.
3. Add an invoice row for the late payment interest and/or compensation. Remember to calculate statutory interest based on the guidelines laid out by the government here.
4. Approve the invoice again.
5. Click on Send reminder.