Thanks for getting back here in the Community, @michella.

There are two possible reasons you're seeing this Overdue amount. First is overpayments for the customers' refund.

Second,  Refund Receipt may have not been used when processing your refund. To do this, here's how:

1. Click + New.
2. Select Refund Receipt.
3. Fill out all the necessary information.
4. Hit Save and Close.

Also, to remove the Overdue amount, you'll have to create a Credit memo and make sure to link it to the check associated with the refund. Here's how to create a credit memo:

1. Click + New.
2. Select Credit Memo.
3. Fill out all the necessary information.
4. Hit Save and Close.

You can go back to your Accounts Receivable section to make sure everything is in place.

Check out this article to learn how to refund a credit card payment in QuickBooks Online: Refund a credit card payment in QuickBooks Online.

Feel free to share your thoughts or concerns here in the Community. I'll be around to back you up. Have a great day and take care always!