According to research by Dave Bell, professor at the Wharton School of Business, customers preferred free shipping worth $6.99 to a $10 discount on a product (via Get Elastic).
If you’ve never picked up a book about the psychology of shopping, hit the bookstore now. It turns out we’re completely irrational about the simplest things involving money.
My recommendation is Robert Cialdini’s “Influence: The Psychology of Persuasion“. It’s valuable as both a marketer and a consumer. We should all be educated about the ways people attempt to influence us.