If Clients have incurred interest, you'll want to include that on their upcoming invoices. Here's how;
The 1st step is to create the expense general ledger account. Here is an image that should help.
The second step is to add a "Product and Service" to facilitate adding this to an invoice.
To include an additional charge as you create a new invoice:
Select the Plus icon (+) at the top.
Fill out the invoice as usual, including the applicable products/services.
In the next unused line, select the Product/Service item for service charges or late fees, or choose Add New under Product/Service and follow the prompts if an item for this type of fee has not yet been set up.
Enter the amount of the charge.
Click the Save and Close button.