Good day, @Marlina.

To add a service charge in an invoice, you must include it as another line item on your transactions.

Here’s how to create an invoice for an additional charge:

1. Go to the Gear icon.
2. Select Invoice.
3. Enter the necessary information in the fields.
4. From the Product/Service drop-down, select Add new.
5. In the Product/Service Information page, choose Service and enter the details for the charge.
6. Then hit Save and close. The charge item will appear on the invoice.

Since our system doesn't provide a default charge amount, I suggest you seek an accountant for expert advice. This will also help you avoid mistakes from your books that will cause extra work.

 
Also adding this article you can use if you need to charge customers for late fees or finance charges: How to enter late fees or finance and service charges on invoices

I'm here anytime you need assistance, just reply to the comment section below.