Having an automatic late payment fee is a great idea, @abimod. However, the feature to have it is currently unavailable.
In the meantime, I recommend sending feedback directly to our product development team. They can consider your suggestions for future updates to add the automatic late payment fees in QuickBooks Online (QBO).
Here's how:
- Click on the Gear icon, then choose Feedback.
- Share your product improvement ideas.
- Click Next to submit.

You can keep track of your ideas or recommendations by visiting the QBO Feature Requests website.
Also, you might want to check this guide to help you record your customer payments: Record invoice payments.
You can always return to this thread if you have any concerns about the late fees.