How to 'Receive Payment' in QuickBooks Online
To 'Receive Payment', on the lefthand menu in QBO, choose 'Create' (the + sign), then under the Customers column choose 'Receive Payment'
Fill in the highlighted info shown below. QBO will show you all open invoices for the customer. Put a checkmark next to the whichever invoice(s) is/are being paid. Verify the amount looks correct and click 'Save and close'.