If your customer hasn't paid you on time for their outstanding invoice, it's reasonable that you'd like to add interest charges. I'll help you achieve that.
Here's a link to another question posted in the Community. This customer had the same question, and the answer provided by a member of the QuickBooks Team was detailed and helpful. Take a look at it here: https://quickbooks.intuit.com/community/Sales-and-invoicing/How-do-I-charge-interest-charges-to-over...
Once you read through that, let me know if you have any outstanding questions. I'll be here to help.