Sample project for secure in-app purchases with Xamarin for iOS and Google Play. It uses Azure functions for server-side receipt validation.
Blog post here: http://jonathanpeppers.com/Blog/securing-in-app-purchases-for-xamarin-with-azure-functions
Google Play is featured on Part 2 here: http://jonathanpeppers.com/Blog/securing-google-play-in-app-purchases-for-xamarin-with-azure-functions