How to test IAB without being charged

I am trying to testIn App Purshase without being charged on Unity by reading this http://ift.tt/1zu5K8S

My application is published as an Alpha version, signed with a real key. I downloaded it with an other Google account that the one I used to publish the app. This account is both added in the testers list of Google Play Service and my app Alpha testers list.

The problem is, when I want to buy an item, I never see the word "test" as the documentation says.

Can someone explain to me which extra steps shall I do in order to be sure to test my In App Billing without being charged?

Thanks a lot.