The PayPal integration makes it easy to bill consumers through their PayPal accounts. When PayPal users click "Buy", they receive the Java app immediately, without ever having to leave the store. Developers get paid as soon as their apps are purchased.
A blog post by Sun's James Gostling reveals that the Java Store Team has been working with PayPal for several months. The new PayPal X platform is what allowed Sun to bring purchasing support to Java Store Beta. PayPal X has driven the development of PayPal's popular services for several years and the company recently announced it would open its APIs to developers.
The main challenge left for the Java Store is global support. Right now the Java Store Beta is only available in the U.S., but Sun says other countries will gain access in 2010. Gostling says, "The framework is in place to go global as fast as the lawyers and accountants can work through the details — but it'll take a while."