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You've reached the right place for help, nelser.
Let me help you handle declined payments for paid invoices in QuickBooks Online (QBO).
Since the invoice was already paid, you can remove the payment record from it. I'll show you how to do it below:
You can also check this article to learn about the common reasons why transactions are being declined and the actions that need to be taken to resolve it: Transaction Result Codes article.
I'm adding the following resources about processing payments for your reference:
Be sure to get back to me if you have additional questions while handling declined payments. It would be my pleasure to help. Have a good one!