How can I determine the gross profit by customer invoice

It's good to see you here in the Community space, @useraccounting15.

In QuickBooks Online (QBO), you can run the Transaction List by Customer report and filter the transaction type to invoices so it'll show the income grouped by customer. 

To pull up the report, here's how:

1. In the left menu, click Reports. 
2. Type in Profit and Loss Comparison in the search box.
3. Set the necessary information you want to show on the report. 
4. Hit Run report. 

You can customize the report to focus on the details that matter. After that, you also have the option to memorize it to save the current customization settings. 

To learn more about managing your financial reports in QBO, feel free to visit our Reports page.

If there's anything else I can do for you when running reports, let me know by commenting below. I'll be more than happy to help. Have a wonderful day.