Thanks for joining the Community space, @Chris 4. Allow me to chime in and share some details about creating interest in overdue invoices. 

You can apply the created service item when you create an invoice for your customer. Just add the service item on the second line. 

In case you haven't created a service item for overdue invoices, you can follow the steps shared by JaneD above. To apply the interest of overdue invoices, here's how:

1. Go to the + New button.
2. Select Invoice.
3. In the Product/Service drop-down list, choose the service item created then fill out the necessary details.
4. Select Save and Send.

You can browse these articles for more details:

• Add product and service items to QuickBooks Online
• Create invoices in QuickBooks Online

Also, you can look for a third-party application that allows creating automatic interest on overdue invoices. Here's how:

1. Log in to your QuickBooks Online account.
2. From the left pane, click on Apps.
3. Search for apps in the search box.

QuickBooks displays the list of applications available that match the description you entered. Once you find the right app, click to open it. We’re working on releasing new features soon. For product updates, you can check out our blog post for recent changes.

I've included these articles that will help you manage your product and services in QuickBooks:

• Change Product and Service types
• Group your products and services into a different category

You can get directly in touch with me if you have additional questions in QuickBooks Online. I'd be more than willing to lend you a hand.