We now have a "Make Payment" button on the email notification that is sent when you send an invoice to a client.  This will then request their login info, then take them straight to the billing page of the project.  We will also be adding this button to the Order Summary email notification that is sent when a project is submitted.  These are the default email templates.  When the Custom email Notification feature is deployed, you will be able to put this button on any email notification.