可以轻松与Rails集成的Web Payment API / Services

What are some of the popular web payment API/services that can be easily integrated with ruby on rails framework? I have tried using PayPal the standard version and it was not too flexibler. I have never tried the pro version. Can you guys suggest other alternatives out there that are easy to use, secure, not too hefty on the commission as well and have What are some of the popular web payment API/se