Sprint has become the third U.S. carrier to agree to direct carrier billing for Android apps published through the operating system’s app store.

Google is currently conducting a phased rollout of the service and expects to reach all Sprint’s customers within a few days, according to a Wednesday post in Google’s official Android developers blog.

The feature will be available on all Sprint Android phones except the HTC Hero and Samsung Moment, Sprint said.

Google’s direct carrier billing allows subscribers to charge purchases made through Android Market to their monthly wireless bill instead of using a credit card.

“We believe that direct carrier billing is a key payment option because it lets users purchase and pay for apps more easily,” Android executive Eric Chu wrote in the post. “It’s also important because it offers a convenient way to buy in regions where credit cards are less common.”

AT&T and T-Mobile USA launched direct carrier billing through Android Market last year. Japan operators SoftBank, KDDI and NTT DoCoMo also offer the service.

Chu says that Google is working with other carriers to expand the availability of the service and said new announcements were expected in the upcoming months.

Yesterday’s announcement of Sprint’s direct carrier billing comes two weeks after Google launched in-app billing, which  lets users buy digital goods like game upgrades without having to exit the app. The move was aimed at helping Android compete more effectively with Apple’s iOS, which also offers in-app payments.