Customers on the Ultimate package, can make a one-time payment to their account from the Billing -> Overview page of the portal:
When you click the “Make a Payment” button, you’ll be able to select an amount to add and a payment method (either by credit card or by Paypal):
Credit card payments will be processed directly; for Paypal payments, you will be redirected to the Paypal website to login to Paypal and process the payment.
The payment will be immediately placed on your account, and can be used right away.