I hear your sentiments, Heather.
I see how great it would be to automatically apply interest for late invoices.
While this option is unavailable in QuickBooks, you may look for a third-party application. I'm unable to suggest one as there are various apps that QuickBooks supports. You can browse this page instead to check for an application that suits your company's needs: QuickBooks Global Apps.
I also suggest sending this product suggestion to be available. You can do so by clicking the Gear icon and then selecting Feedback. You can enter Automatically apply interest for late invoices as your keywords. Hit Next.
Your feedback goes to our Product Development team to help QuickBooks have a more friendly user interface.
You can also browse this article as your guide in keeping track of your customer payments: Record invoice payments in QuickBooks Online.
Fill me in if you have concerns about recording transactions. I'm here to help you out.
