Hello, @NelsonDFS.

It's nice to have you here in the Community, I'd be glad to help you today.

If you're referring to adding a line in between the items when creating invoices/estimates, you may need to click on the last box/line under Product/Service. Let me show  you how.

1. Go to the Plus (+) icon.
2. Select Invoice under Customers.
3. Fill in the necessary information.
4. Click on the last box/line under Product/Service to add a row.
5. Add the product/item and amount.
6. Click Save and close.

Let us know if you're referring to something else by adding a comment below, we're always here to help. Have a great day ahead!