Where do I go to post a customers payment towards their invoice

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.

1. Select the Plus sign.
2. Click Receive Payment.
3. Select the person in the Choose a customer field.
4. Enter payment information in the Payment date, Payment method and Amount received fields. 
5. Select the Invoice to apply the payment to if there are multiple and the desired one didn't default.
6. Click Save and close.

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.