I am trying to incorporate Google Play inapp v3 to my most popular game. In a first version, I just one to add an inapp item so the ads could be disabled, thus I can remove the pay version of my app and have only one unificated.
The inapp documentation from Google is good enough for making most of the basic things, although the procedure is kind of complicated if you want to be picky with security, but it is something natural.
What I really hate and I think they should improve, is the testing process. There are two ways of testing: with static responses, or using your own products identifiers. The static responses just uses a series of reserved product ids that lets you force several different responses from Google Play app so you can check that all of them are managed as expected.
The problem start when you want to test things using your own products identifiers, i.e., when you want to do real testing.
You will need a testing account because you cannot make actual inapp purchases of your own products with your actual developer account. So if you do not have already one, you should create a new gmail account for these things. This is not a major problem: creating a Gmail account is free, and easy. The real problem is that, as far as you cannot test inapp from an emulator, you need to use an actual device and set your testing account as the primary account of such device, what implies a factory reset of the used device.
So you will surely need a dedicated device for testing inapp, which is not a major problem for a company with a reasonable budget, but sure it is a problem for many indie developers. Not to talk about testing it in several different devices...
You will need to upload your apk as a draft to Google Play, and you will only be able to use inapp with that same apk installed on your testing(s) device(s). That means that for every change you want to do during testing, you will need to export a new signed apk and upload it to Google Play.
I think Google Play engineers could improve this testing process a lot, helping a lot of independent developers to incorporate Google Play inapp to their apps without spending a lot of time in testing administrative tasks, and without the necessity of a dedicated device. And I hope they will do it...