FREE is unrationally powerful!

Via NeuroMarketing:

When they [Amazon.com] launched a “free shipping” promotion with the purchase of a second book every country except France showed a big jump in sales from the offer. The Amazon marketers investigated, thinking perhaps the French were rational enough not to be swayed into buying a second book. In fact, they found that in France the program had been slightly altered. Instead of zero shipping, the offer in France charged a mere one franc - about twenty cents. From a pure economic standpoint, the two offers are almost indistinguishable. In actual performance, though, the one franc offer caused no sales increase. (When the French offer was changed to FREE!, sales did indeed jump.)