The only comparison that’s relevant to the argument is between the cost of an item when mail-ordered with points and the cost of the same item when purchased in a store. Other credit-card companies’ prices are irrelevant.

Whether or not points can be used on other items is irrelevant to the question of whether mail-ordering using bonus points is cheaper than buying the same items in a retail store.

The conclusion is limited to customers using the points, so these customers have met the eligibility requirements, whatever they are. The argument doesn’t rely on any assumption about how difficult it is to obtain points.

This must be true for the conclusion to follow. If (D) weren’t true, then shipping costs would make mail-ordering with bonus points more expensive overall than purchasing the same item from a store.

The argument doesn’t need the store prices to be higher than the suggested prices; it just needs them to be higher than the mail-order costs.