I'm reading the chapter about "free". And I'm not sure their candy experiment is very good. Comparing 1 cent candy and free candy is not really fair. Most of the cost of 1 cent candy is in the friction of looking for 1 cent, while you can just grab a free one.
Ok, I just needed to read a little further😁 Turns out they controlled for it by selling candy in cafeteria so that their cost would just be added to the bill total.