I need assistance on posting an overpayment, and recording a refund for the same overpayment.

I'll help you track the overpayment accurately, usernatasha.

 Let's start by recording the refund for your customer. Here's how:

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

Once done, let's link the refund to the customer's credit or overpayment. I'll show you how:

1. Go to New, and select Receive payment.
2. Select the customer you used for the check or expense when recording the refund.
3. Fill out the other fields.
4. Under the Outstanding Transaction section, select the checkbox for the Expense or Check you created when recording the refund.
5. Make sure the payment is equal to the open balance, then select Save and close.

For more details, please see this article: Record a Customer Refund in QuickBooks Online.

Additionally, I'd recommend reconciling your accounts every month. This will help you monitor your income/expenses and detect any possible errors accordingly. You can check out this article for the detailed steps: Reconciliation Workflow.

Please don't hesitate to let me know if you have other concerns about managing invoices and payments. You can drop a comment below, and I'll gladly help. Take care, and I wish you continued success.