iPhone and iPad users are familiar with this process that lets you pay for options, services, subscriptions, and virtual goods from within the app itself. This feature appeared in iOS last summer when iOS 4 was launched along with the iPhone 4.
It is interesting to note the different ways Apple and Google think about this feature from the language they use. Apple calls it in-app purchasing which can be interpreted as focusing on the end-user (customer) experience. Google calls it in-app billing which I interpret as a developer focus.
Several apps take advantage of in-app billing/purchasing now. This includes Tapulous’s (Disney Mobile) Tap Tap Revenge game and ComiXology’s Comics app. The apps themselves are free. Content for apps cost real money (in-app purchasing).
There free digital comic books available in the ComiXology app to let you get a feel for it without spending any hard earned cash. Having tried the iOS version of the app on both the iPhone and iPad, I can tell you that while the experience on the smaller phone screen is as good as you can expect (the best I’ve seen on phones), the experience is much better on the iPad’s larger screen. That said, ComiXology has done an amazing job of presenting comic books panel by panel on phones and full pages on tablets (the iPad).