Additional IP addresses can be purchased during your initial server purchase for $2/month per additional IP address.
If you require additional IP addresses after your initial purchase, from the billing panel, please first login to your account. Once you have logged in navigate to your services page by selecting the dropdown menu item "Services" and click on "My Services". From this page, select the service you are wanting to purchase additional IPs for, you will be taken to the overview of that particular service. On the left hand side select the menu item "Upgrade/Downgrade Options". This will bring you to a page where you can select additional IP addresses to add to your plan.