Client paid Invoice online, however, Invoice was incorrectly sent to them. Too late to refund payment. What do I do?

It's nice to see you here in the Community, @Uhm_ok.

Let me share the steps on how you can remove the invoice and keep track the payment in QuickBooks Online.

You can void the invoice created for that specific client. Once you void the invoice, you still have a record of the transaction, but it won’t affect your account balances or reports.

Let me guide you how:

1. Go to the Sales menu on the upper right side. 
2. Choose the Customer tab.
3. Locate the customer's name, then double-click the invoice.
4. In the Invoice window, click More at the bottom.
5. Choose Void, then click Yes to confirm.

For more information about voiding invoices, please visit this article: Void or delete transactions in QuickBooks Online.

You also have the option to see the voided transaction by going to the Report tab. Search Transaction List by Customer, then customize the report.

Once done, you can now apply the credit to their invoices. For more information, see this article: Create and apply credit memos or delayed credits in QuickBooks Online. It also contains steps on how to turn on/off the automatically to apply for credits option.

I'll be adding these handy articles that will guide you in recording payments and how to personalized your sales forms:

• Record invoice payments in QuickBooks Online
• Customize invoices, estimates, and sales receipts in QuickBooks Online

Let me know if there's anything else I can do for you. Have a great day!