Once an invoice has been selected in the "Invoice ID" field, the biller name, customer name. total of the invoice, amount already paid and the amount outstanding will be displayed in the "Details" section of the Process Payment screen.
If you dont see any information in the "Details" section either you haven't selected a valid invoice in the "Invoice ID" or you have not entered the invoice ID correctly. Please refer to this page on how to enter the invoice ID correctly.