Can I retroactively add a project name for a repeat customer to a paid invoice, and if so, how? I want all project costs to be properly assigned.

Yes, you can, @krzpy9rq612g.

I’d be more than happy to help you record this in your QuickBooks Online (QBO) account. Here’s how:

1. Go to the Expenses menu.
2. Select the Expenses tab.
3. Find and open the transaction.
4. Select the project from the Customer/Project drop-down menu. 
5. Then click Save and Close.

You can also check Step 4: Add existing expenses and timesheets to projects section in this article for more details about this: Set up and create projects in QuickBooks Online.

Once done, you can now track your project cost and profitability information.

Let me know if there’s anything else I can help you with managing your projects or regarding your account. I’ll be around ready to back you up. Stay safe and keep well.