Helping you in recording the extra invoice payment in QuickBooks Online (QBO) is my priority, @In Awe. Let me share further details and guide you with the actions you need to take care of this matter.

When a customer (unintentionally) paid more than what they owed, issuing a credit memo is the best option. You can bypass creating a sales receipt (which never happens) since you can use the credit memo to reduce the balance on your customer's next invoice.

If your customer wants a credit, you can create one by following these steps:

1. Go to the  + New button.
2. Select Credit memo.
3. Chose the customer's name in the Customer dropdown, then enter the credit memo details, such as the date and the amount. You can also create a custom credit service item to add it to credit memos as a single line item.
4. When you're done, select Save and close.

Then, you can also turn on the credit memo auto-apply feature (if you haven't already). This lets QuickBooks automatically apply credit memos to customers' balances or open invoices. You can turn off this feature anytime if you want to decide which open invoices you want to apply credit memos to.

Once you're done, go ahead and apply the credit memo to an invoice (if the credit memo auto-apply feature is turned off). Here's how:

1. Select + New, then choose the Receive payment option.
2. In the Customer dropdown, select the customer.
3. In the Outstanding Transactions section, select the open invoice you want to apply the credit memo to, then select the credit memos you want to apply.
4. For the open invoice in the Payment column, enter how much of the credit you want to apply.
5. Leave the Payment method, Reference no, Deposit to, and Amount received fields blank.
6. Fill out the rest of the form, including the Payment date.
7. Make sure the total is correct after applying the credit memo, then click Save and close.

To learn more about how and when to give a credit memo to customers in QBO, you can refer to this article: Create and apply credit memos or delayed credits in QuickBooks Online.

If your customer wants a refund, you can refer to this article for the complete guide: Record a customer refund in QuickBooks Online.

Also, I'd recommend pulling up customers and invoices (i.e., Transaction List by Customer and Invoices and Received Payments) reports. This will help you in monitoring your sales and income transactions. To do this, visit the Who owes you and Sales and customers sections from the Reports menu's Standard tab.

Please keep me posted in the comments if you have other concerns about managing invoices and payment transactions in QBO. I'm always ready to help. Take care, and I wish you continued success, @In Awe.