A certain credit-card company awards its customers bonus points for using its credit card. Customers can use accumulated points in the purchase of brand name merchandise by mail at prices lower than the manufacturers' suggested retail prices. At any given time, ttherefore, customers who purchase merchandise using the bonus points spend less than they would spend if they purchased the same merchandise in retail stores.
The assumption that the amount credit-card customers pay for shipping the merchandise ordered by mail does not increase the amount customers spend to an amount greater than they would spend if they purchased the same merchandise in retail stores is on which the argument depends.