Hello, @catbagsic. I'll guide you on how to add an invoice to a project in QuickBooks Online (QBO).

Before we proceed, please know that when you attach a paid invoice to a project, the status of the entry switches to unpaid. It's also separated from the attached payment. To avoid unintentionally billing your customer twice, make sure to link the payment to the invoice after editing it. 

To add the paid invoice to a project, here's how:
 

1. From the left menu, select Sales.
2. Choose Invoices.
3. Look for the paid invoice you want to add to a project.
4. Click Edit.
5. Select the Customer drop-down, then choose the project.
6. Once done, click Save.
    

After completing the steps above, let's reattach the payment. 

Additionally, if you wish to run a project report in the future, you can follow the steps below:

1. Navigate to Reports.
2. In the search bar, type in Project Profitability Summary.
3. Choose Customize.
4. Modify the Report period.
5. Enlarge the Filter section.
6. In the Project Status, choose In Progress.
7. Select Run report.
    

Please leave comment below if you have follow-up questions about adding an invoice to a project or any other concerns within QBO. We're happy to help. Have a good one.