It's an honor to be given the chance to assist you today, pauldoyler-gmail.
You van apply a credit note directly to open invoices in QuickBooks Online (QBO).
Let me to show you how to do it:
Go to the + New button and select Credit note.
Select the Customer. From the To Link Invoice window, click Add.
Ensure that the details are correct. Tap Save and close.
Once done, you'll want to pull up the Transaction List by Customer report. This will show all the customer transactions you've created including the credit notes. You can search it on the Reports menu. If you need to edit it, please check this link: Customise reports in QuickBooks Online.