Trialy makes it easy to create a free trial for your in-app-purchases. Trialy takes care of the heavy lifting for you (using server-side checks with customizable grace periods to allow for offline use, for example) so you can focus on developing an amazing app. In addition to configuring the trial length, you can decide whether users should be able to try your app on each of their device, or just once for each Google account.
The in app purchase system provided by Google is missing one very important thing. This tool will allow app developers to provide their users with codes to be redeemed in app for extra content.
This system uses Parse.com as a simple and free backend to manage your users and codes. All you need to do is create a free account and setup your app. Once you have your client and application keys, you are ready to go.
android-checkout is a library for Android In-App Billing (v3). The main goal is to reduce work which should be done by developers who want to integrate in-app purchases in their products. The project is inspired by Volley library and is designed to be easy to use, fast and flexible.