You can create a service item for the interest charge and add it to your customer invoices in QuickBooks Online (QBO), Jsurka. We'll expound more details below to help you with this task.

There isn't a direct option to add interest on a customer's invoice. However, you can create a service item and include it in the customer's transaction to indicate that it's for the outstanding amount. Doing this will ensure the system recognizes the late fees or charges on the invoice transaction. We'll outline the steps to get you going:

1. Access your QuickBooks Online company.
2. On the top right corner, click the Gear icon and select Products and services.
3. Click the New button and select Service.
4. In the Name field, enter Late Fees or Late Charges.
5. From the Category ▼ dropdown, select the category that best describes the fee.
6. Complete the remaining fields, as you feel necessary
7. When you're ready, click the Save and close button.

For visual reference, see the screenshot.

You can also check this article for further guidelines: Add product and service items to QuickBooks Online.

Moreover, here are some articles about creating personalised sales forms and generating a report to keep track of transactions inside the program:

• Customise invoices, quotes, and sales receipts in QuickBooks Online
• Run reports in QuickBooks Online

In case you require additional support with customer invoices or payments in QuickBooks, please visit us or comment below. We're here for any other software inquiries. Stay safe!