In-app Purchase

In-app Purchase Walkthrough
http://troybrant.net/blog/2010/01/in-app-purchases-a-full-walkthrough/

Appcelerator Storekit doc
http://docs.appcelerator.com/titanium/latest/#!/guide/StoreKit_Module_In-App_Purchase_Testing


Apple official doc.
http://developer.apple.com/library/iOS/#documentation/NetworkingInternet/Conceptual/StoreKitGuide/Introduction/Introduction.html


Urban Airship (Not free)
http://urbanairship.com/products/in-app-purchase/

Urban Airship manages device verification and content delivery of mobile purchases. After a user makes a purchase decision, billing is handled through the user’s iTunes account and content is delivered directly to the app. Once you set up your platform IAP system (Google Marketplace/iTunes Connect), we will host and deliver content and secure receipt verification, replacing your need for an IAP server.