Thanks for posting in the Community, @rick9. I'm here to help with posting a customer's payment towards their invoice.
Here's how you can apply a payment towards an invoice in QuickBooks Online.
The following is a video to serve as an example:
I recommend the following article which covers this process and includes information on other payment ,ethods, such as how to group them:
How to Record a Customer Payment.
That's it. I'm confident that you'll have the payment applied in no time.
Don't hesitate to let me know if I can be of further assistance to you. Take care.