Once you have logged in to the Portal, go to Account> Invoice
When you are at the Invoice page, you will see the list of invoices for your account.
If there is an unpaid invoice in your account, that particular invoice will have a "Pay Online" button.
*You will need to log-in again by using the same credentials as the Portal.
Once you click on the "Pay Online" link, you will be redirected to the Invoice page with the option of paying via credit card or Paypal from the dropdown list.