The author concludes that customers spend less when they use credit card points to mail order merchandise than when they buy the same items in store. This is because when customers order items using points, the prices of those items are lower than the manufacturer's suggested retail prices (MSRP).

Since we're looking for a necessary assumption, we have to find any gaps where the author's conclusion doesn't necessarily follow from the premises—meaning reasons why the conclusion might be false even though the premises are true. In this case, that means we're looking for reasons why consumers might not pay less using points even though the merchandise is sold at lower prices than the MSRP.

One reason is that the author's conclusion is about the prices in the points catalogue versus retail prices, but the premises are only about suggested retail prices. Since we don't know from the premises whether retailers actually follow the MSRP, the author must assume that retailers don't sell products for less than MSRP, at least not by enough to match the points catalogue prices.

Even if retail prices are actually higher than points catalogue prices, we also don't know that the price of the item is the only cost involved. If there are significant shipping fees or other costs to mail-order these products, then consumers might not spend less using points even if the face price of each item is lower. So the author also has to assume that purchasing by mail order doesn't have extra costs that outweigh the price difference.

We've identified two necessary assumptions, but the correct answer doesn't have to include both (or indeed either) of them. We're not trying to make the argument valid—that's too high of a bar. So as long as the correct answer targets one single necessary assumption, that's good enough. It can even identify a necessary assumption that we haven't anticipated.