Thanks for visiting the QuickBooks Community, @john.berry.
Allow me to help you apply a credit note to your customer’s invoice in QuickBooks Online.
Here's how to manually apply a credit note to an invoice. This process gives you control over which open invoice you can apply a credit note to.
The steps above will help you apply the credit to your customer’s open invoices. For your reference, you may check out this article: Apply a credit note or create and apply a delayed credit.
Please let me know if you have more questions. I’ll be here to help. Wishing you the best!