Here’s how I implemented in App Billing in my app(s).
My goal was to implement a donate feature for my wallpaper apps so I set about reading the documentation on the Google developer pages.
Events took a familiar course: after 10 minutes of reading, I thought, there’s got to be a less painful way to do this. So I googled the magic words tutorial and found quite a good one.
However, the recommended method given by Google is annoying. You have to copy Java classes around by hand and there are no Gradle dependencies to make things easy. I got it working in the end, but was unsatisfied.
Then I found this Github project: Android In-App Billing v3 Library. Do youself a favour and just use this. You can install it using Gradle and everything you need to know is in the sample project.
If you have questions, do ask below.