I am not sure about this...

"Noah Kagan (founder of AppSumo) has this thing he calls the “coffee challenge.” Basically you walk into a coffee shop, order whatever it is you want, and when it comes time to pay, ask for 10% off. If the cashier asks why, say “just cause.” Most of the time, the cashier will just give it to you."

This rarely works. I've tried.

isn't the lesson, don't fear rejection? 

Yes, but also don't be surprised when you are rejected. 

I think you need to have this innate feeling that you are special and thus deserve a discount (or any special treatment).  I have seen people do that, and for them it works - almost always.  I am not sure how to develop this :)

Practice, Masha. The more you practice, the more innate it becomes.