I can definitely help you with this, michella.

QuickBooks Online offers 3 ways of handling customer overpayments. The first method is applying the credit to an invoice which is not applicable in your case. The second one is recording it as a refund. In this process, we can use a check or expense to record the refund that will reduce your bank's balance and offset your customer's open credit. Check out the steps that I've listed below.

First, we'll have to record the refund for your customer:

1. Click the + New icon.
2. Select Expense or Check. 
3. Select the customer you want to refund from the Payee ▼ drop-down.
4. From the Payment account ▼ drop-down, select the bank account where you deposited the overpayment.
5. On the first line of the Category column, select an Accounts Receivable.
6. Enter how much you want to refund in the Amount field.
7. Fill out the other required fields.
8. Hit Save and close.

Then, let's link the refund to the customer's credit:

1. Click the + New icon.
2. Select Receive payment or Receive invoice payment.
3. Choose the same customer you used for the check or expense.
4. Fill out the required fields.
5. Under the Outstanding Transaction section, select the checkbox for the Expense or Check you created.
6. Click Save and close to complete the process.

The last option in handling a customer overpayment is recording it as a tip. However, this is only used if your customer paid more than their total on purpose. You can refer to this article to learn more about this process: Record an invoice overpayment as a tip.

In case you want to track all the refund receipts that you've processed for the year. you can follow these steps in customizing the Transaction List by Customer report:
 

1. Select Reports from the sidebar menu, then enter Transaction List by Customer on the search bar.
2. Click the Customize button.
3. Select the appropriate reporting period.
4. Under Filter, select Refund from the Transaction Type drop-down menu.
5. You can choose a specific customer from the Customer drop-down menu, then Run report.

Do you any other questions in mind? Feel free to leave them below and I'll make sure to get back to you as quickly as I can. Have a great rest of the day michella!