Chargify is a great solution with one simple objective in mind: to help you to does the billing stuff for your site. Only take care of your main service or product and Chargify do the rest. it keeps your billing information safe and ready to use. Chargify has an API to integrate the billing solution with your project.
Venture Slogan: Build Your Business, Not Your Billing System.
Chargify’s own description: “…Chargify is a recurring billing application developed by Grasshopper Labs. Chargify simplifies recurring billing for Web 2.0 and SaaS companies…”