When I released Jaikoz five years ago (!) Paypal was the sensible way to go for processing payment. A few years later and some potential customers did not want to use Paypal so I added Google Checkout as well, but I didn't have time to fully automate the Google process.
Yesterday I decided to remedy this, the API and documentation for Google Checkout is way better than Paypal. Both are comprehensive and easy to use and I got things working within the day. I went back to the Paypal website to see what they are offering now, there was now a Java API but it achieves the dubious honour of being both limited and totally confusing.
Google also takes a smaller cut than Paypal so if you are buying a Jaikoz license please use the Google method.
So my point is integrating with Google is easier than other solutions, and this is one reason why I think Android will overtake iPhone within the year