I ended up writing my own app to sell my ebook and it wasn't all that hard using Stripe. Deploying to Heroku is free if you're ok with their wildcard https certificate.

<shameless plug> The book is called Mastering Modern Payments: Using Stripe with Rails. It's a guide to integrating Stripe with your Rails app and includes a bunch of goodies that help make your integration robust in the face of failure.

http://www.petekeen.net/mastering-modern-payments </plug>