When an unemployed stoner scores a bag of weed for you and then takes a portion out for himself before delivering the bag. This serves as a) compensation for the person's time and effort and as b) a reason for said unemployed stoner to find the motivation to drive across town and score a bag - sometimes even in perilous and hazardous driving conditions.
Ah, man, my bag's a gram short! Damn, I guess I got hit with the 'Robin Tax' again! Oh well, at least he didn't charge me for gas!