On the top right of your Site Settings page is a button that says Activate Subscription. The popup that comes up after you click that button calculates your cost for checkout, including add-ons services. If you simply want to preview your cost, visit http://designedtorun.com/#pricing
Add-on services can also be paid for individually by clicking the Purchase buttons under the Optional Add-ons header, on the Billing tab of the Site Settings page.