Glad to have you here in the Community, DR3786.
Let me share some insights about refunding a customer overpayment in QuickBooks Online.
For now, we can only process refunds for sales paid with credit cards. For those that were paid using ACH, you can deposit the money back to your customer's account using other methods. Then, record the transaction in QBO.
Here are some articles you can refer to for future guides about refunding customers for credit cards:
Please reach back out to me here with any additional information on the issue, @DR3786, so I can get you taken care of. Have a good one!