I have one, simple question: Is robotmedia's inAppBilling library reliable and eligible to use, or would you rather suggest me classic inAppBilling imlementation?
Given that robotmedia's library you link to hasn't been updated for 7 months (At time of writing), and specifically states it only supports V1 and V2 of GooglePlay In-App Purchase, I would suggest the Google implementation which currently runs on V3, simply because of the more apparent update cycle.
V3 improves the usability making the whole system much easier to integrate with your application, so in terms of reliability I would put forward that you are less likely to introduce bugs of your own in a simpler implementation.
There are plenty of resources showing you how to set up and get In App Billing working with Android through the Google Play libraries. (RobotMedia also as a tutorial themselves for their version).
You can read about implementing Google Play Services In App Billing here.