Create in App Billing Function for my App

I have a number of apps launched in Android. The main one is a paid app and I have a few spin off versions that are free. For the paid app I also have a "lite" version that is free but with limited functionality. I need to create the in app billing function whereby if someone wants to unlock fucntionality and upgrade to the full app they can purchase this within the app. This will be a one time occurrence for the user so I do not envisage creating a server system to keep all the purchase data but it should all happen within the application. The project is therefore to create the In App billing functionality so it is up and running and works in the Lite version - then install it also in 4 other apps - where there is also the chance to upgrade to the one main app that I mentioned above.