Is the following question correct or incorrect? (Explain your answer)

A store provides the following offer: When you purchase $40 of goods, you will be given a $5 coupon, which can be used in the next purchase of $40. This offer is only provided once to each customer. A student think that it is a great offer because one can have a $5 discount once in the first purchase and again in the second purchase. (Suppose one can exactly purchase goods of $40 twice from this store.)