Recurring Billing for Web 2.0 and SaaS Companies – Chargify

How does Chargify work with my existing website?

You have two options for integrating Chargify into your existing website. 1) The quickest and easiest way is to link your website to a custom branded payment page to sign up for your product or service. 2) For the more technically inclined, integrating our API gives you the ability to seamlessly integrate Chargify into your applications.
(Link: Recurring Billing for Web 2.0 and SaaS Companies – Chargify)