How do you bill your clients? (for your app)
callmeed: If you're doing it in Rails, I'd recommend the ActiveMerchant plugin and a gateway/merchant account from Braintree.That's what we use on our main Rails app (as opposed to PayPal on our other main subscription-based offering). It works great. The only hangup has been accepting international currencies (we're having to work with a different company for that).The transaction fees are low, too–which actually makes a difference when you start getting a lot of subscribers.http://www.braintreepaymentsolutions.com/
http://www.activemerchant.org/
https://peepcode.com/products/activemerchant-pdf