It's good to see you here in the Community!

At this time, there isn't a way to add another line in the Invoice Details area. 

I can see that this option is helpful to you and your business to provide more details on the invoice. For now, I suggest submitting feedback directly to our product engineers. This way, they may look into this suggestion and consider adding the option in future updates. To submit feedback, you can click the Feedback button and enter your suggestion. 

Want to review your transactions in QuickBooks Self-Employed? You can go to the Transactions menu and filter the date you want to see.

It’s nice to work with you! Don't hesitate to click the Reply button below if you need further assistance creating invoices in QuickBooks Self-Employed.