Amazon is preparing the latest salvo in its fight to create an alternative app store to Google’s Android Market, but it’s not quite clear when the online retail giant intends to fire it.

That salvo is in-app purchases, a feature that, by all rights, would be a big addition to the features it offers in the Amazon Appstore. In-app purchases have been available to many developers through an invitation-only beta program for some time, as PocketGamer reports, but the feature hasn’t been opened up to all developers. However, the report cites unnamed sources who suggest that Amazon could be about to change that, perhaps as early as this month.

Though it’s only available in the U.S. and has seen some controversy over pricing and revenue paid to developers, the Amazon Appstore has picked up a fair amount of popularity and makes for a fairly strong alternative to Google’s market. But one thing that Google offers and Amazon doesn’t is in-app purchasing, and for many developers – especially game makers – that can make a huge difference.

In-app purchases, or micro transactions as they’re often called, can be extremely important to making money in the mobile game market. Downloads themselves don’t always drag in a ton of revenue for a developer, but many players will commit to spending money to buying in-game power-ups and other boosters. A good example, though it’s on iOS, is Epic Games’ popular Infinity Blade, which had a serious revenue uptick after it added in-app purchases to its already well-selling game. And that’s to say nothing of the entire genre of games known as “freemium,” which are free to download but often include (or require) in-app purchasing.

Without in-app purchases, developers may well be leaving money on the table. And then there’s the fact that Amazon’s seeing some serious success in the Android sphere. Its newly released Kindle Fire tablet took the No. 2 spot in tablet sales behind the iPad in the final quarter of 2011, and all those tablets have the Amazon Appstore on the home screen as their download portal. That’s quite a bit of money Amazon and developers could be making.

Amazon is likely to be working to bring itself in line with other app stores as soon as possible, but in the meantime, developers interested in finding their way into the IAP beta for the Amazon Appstore can inquire here.