Yep. Especially if you want people to act on advice. If you give it for free they’re likely to disregard it. But if you charge where it’s an uncomfortable number; they’ll implement your advice in a hurry. 

 Funny how humans work

Love learning about user behaviour as such, more so than even the interface part 

 Behavioral economics and psychology are some of the most fascinating things to study. They reveal a lot about human nature and deepen your understanding of the world. Highly recommend.