Does anyone know how I can add a line in the "Invoice Details" area? I see it has invoice number, date, and due date, but I need another line there for product number.

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.

If I am understanding you correctly, you want to add a box on your invoices for "product" and a box for the number?

If so, open your invoice go to formatting to customize data layout ....many of the templates have an option for "other" that you can name Product.

You might have to click on layout designer to move your new product entry and box someplace quickbooks is happy with