One of the most sought-after features for the Android platform has been the ability to charge app users for things they buy inside your app. In-app billing is an important part of the Apple iOS environment and a big reason why iPhone app development attracts so many programmers.

Google (Nasdaq: GOOG) has been promising its own in-app billing system for some time, but the release schedule was always nebulous. But that's all in the past now: Android apps can start charging for content.

On Friday, App developers were told to expect the in-app billing feature to start working this week. The tools have been available since early February, but there was no way to test their functionality. At the moment, developers can run transactions through testing accounts, to make sure everything works when the billing actually goes live in a few days.

Of course, this new feature comes only days after Amazon.com (Nasdaq: AMZN) introduced its own Android app store and gives Big G a leg up on the new competition -- at least until Amazon presents a similar feature of its own.

And here's where things get tricky: Will an app that was published in and downloaded from the Amazon store be able to run Google's in-app buying code? Google gets 30% of all in-app revenue, so allowing cross-store buys is good for the pocketbook. Furthermore, you'd need a presence in the Google-approved Android Market to pull that off, so you can see this as a way to keep developers from running willy-nilly to other stores and leave the official one abandoned -- even if the new destinations are much better in many ways.

So add-on media content and fully featured FarmVille apps for Android can't be far away now. It remains to be seen how user-friendly the in-app buying experience is, as well as how quickly users take a shine to it. If all goes well, Google has another method for monetizing mobile apps as well as a reason for developers to stay loyal to the Google-approved app store; if not, I foresee many versions of this feature over several years until Google finally gets it right.

Will Google realize the potential benefits of this store-in-a-store? Discuss in the comments section below, and make sure to add Google to your watchlist so you'll know how it works out.

Fool contributor Anders Bylund owns shares of Google but holds no other position in any of the companies discussed here. Google is a Motley Fool Inside Value pick. Google is a Motley Fool Rule Breakers choice. Apple and Amazon.com are Motley Fool Stock Advisor selections. The Fool has written puts on Apple. Motley Fool Options has recommended a bull call spread position on Apple. The Fool owns shares of Apple and Google. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. You can check out Anders' holdings and a concise bio if you like, and The Motley Fool is investors writing for investors.