I can see how you want this feature to be added in QuickBooks. I'll guide you on how you can add interest on past due invoices, Heather.

For now, applying interest automatically on past due invoice invoices isn't an option. As an alternative, you can manually add them directly on the invoice. To do so, you can follow the step outlined below:

1. Go to the Sales tab
2. Under All sales tab, locate for the overdue invoice and click Edit.
3. Go to the line item where you want to add the interest. 
4. Under the Product/Service field, click the drop-down arrow and click Add new.
5. On the Product/Service information, click Service.
6. Enter the name of the service like Interest.
   
   
   
    
7. Tap Save and close.

Furthermore, QuickBooks downloads the transactions automatically. Learn how it looks for and tries to match them with those you've already entered in your account, I've added this article for more details: Categorise And Match Online Bank Transactions In QuickBooks Online.

Please get back to us if you have any questions when adding interest. As always, we're here to ensure everything is taken care of.