Android users on Sprint's U.S. network will soon be able to bill Android Marketplace purchases directly to their Sprint accounts, simplifying the process for buying apps including games. AT&T and T-Mobile started offering the direct billing option for Android users in late 2010, but Sprint's phased roll-out of the service -- which will complete in the next few days -- brings the option to approximately 3.5 million Android users who use the carrier. Direct billing for app purchases is still not available on Verizon, which provides service for about half of the 14 million or so U.S. Android users, according to recent market research. "We believe that Direct Carrier Billing is a key payment option because it lets users purchase and pay for apps more easily," Android's Eric Chu said in a Wednesday blog post announcing the Sprint roll out. "It’s also important because it offers a convenient way to buy in regions where credit cards are less common." Direct carrier billing for Android is also available on three major Japanese mobile networks, where cell phone accounts can often be used for purchases at convenience stores and vending machines as well. Payment provider Zong recently saw a major expansion of its cell phone account-linked payment service, which lets developers add mobile billing payment options to all sorts of mobile and non-mobile games. Google recently began offering official support for in-app Android purchases using an existing Google Market account, obviating the need for third-party systems like those from Zong or Paypal for such purchases. The company is also reportedly looking in to exercising "non-fragmentation clauses" in its contracts with carriers, in an effort to unify the Android user experience across multiple heandsets and service providers.