Another option to take a look at is http://recurly.com/ http://recurly.com/ We offer a super simple RESTful api that lets add payment functionality to your site

Another option to take a look at is http://recurly.com/ http://recurly.com/ We offer a super simple RESTful api that lets add payment functionality to your sites. We also handle all the complexities of upgrades/downgrades and subscription management. We're here to help :)