Good day, harpspools-gmail.
You can use the receive payments and select the invoice you want to pay. Then, enter the actual amount paid, and QuickBooks will ask if you want to keep the over payment as a credit. This way, you can apply the amount to the customer's future invoices.
For more information on how to handle customer overpayment, you can check out this article: Handle a customer credit or overpayment in QuickBooks Online.
Please let me know if you have more questions about customer transactions in QuickBooks Online. I'm always around to answer them. Take care and stay safe.
