Hansel and Gretel Marketing - How Barneys Entices Customers to Come Back

This and other sales tricks are explained in Cialdini's "Influence: The Psychology of Persuasion".

I really like that book because it made me feel more conscious about those tricks, even though it's mostly stuff "everybody knows".

"The tactic works pyschologically to make her feel indebted"

I've had similar feelings toward free offers and I'm sure it's a common occurrence.