I'll share some details about adding invoice interest to the QuickBooks Desktop, @HeatherMarie1.  This way, I can guide you accordingly in handling your sales transactions. 

Adding interest in the invoice in QuickBooks Desktop is the same as QuickBooks Online, that you need to create a service item. Then add it to the second line when creating the invoice. 

In case you still need to create a service item for the overdue invoices, you can follow the steps below: 

1. Go to the Lists and select Item List. 
2. Press Ctrl+N and pick Service in the Type drop-down. 
3. Enter an Item Name and complete the Rate, Tax Code, and Account. 
4. Hit OK when done. 

For more details, see this link: Add, edit, and delete items.

Once done, add the service item you created in the second line in the invoice form. See the sample screenshot below for more reference: 

Moreover, check out the following articles to learn how to personalize your sales forms and how to prevent a finance charge from being assessed on an invoice:

• Assess finance charges
• Use and customize form templates.

Feel free to get back if you need an extra hand managing your invoices. I'll be glad to assist you. Take care!