There maybe times when you have an order with an open balance. Making payments towards that balance is easy.
When you are logged into the portal, if you have a balance you will see a yellow bar along the top of the profile. You can either click View Orders there or scroll down to Purchase History.
On the Orders page, find the order you are wanting to make a payment towards then click View Order.
On the Order Details page you will see the product(s) for the order, any payments already made towards it, as well as the remaining amount owed. On the right side, enter in the amount you would like to pay, enter in your payment details and then click on Submit Payment.