[fdiotalevi]

At the moment I'm using Paypal. The documentation is insufficient and code samples (at least the Java one) are horrible, but it gives you the ability to accept credit card payments (without requiring the user to sign up for a Paypal account). Unfortunately that's not the case for Google Checkout, which requires a google account.

My next step is Braintree http://www.braintreepaymentsolutions.com/ Its API is much better, and it's more flexible than Paypal. It has some fixed costs, but nothing too bad if you are serious about your business.