The manager is not comparing past costs to potential future cost. The conclusion could be true regardless of whether this is the most expensive season to offer gift-wrapping.

There’s no information in the stimulus about what slows down shoppers. The closest the manager comes to addressing this is “time-consuming for us,” but we can’t assume that extends to the customer as well.

(C) is in line with the argument, but it is not necessary that charging benefits the store. Even if there was no benefit from charging, it could still be true that there’s no reason to offer it for free.

This is another reason to not offer free gift wrapping, but it isn’t necessary for the conclusion. Whether or not it’s expensive to inform the customers, the conclusion could still be true.

This must be true. We needed to know that the two possibilities addressed in the manager’s argument are the only two possibilities, and that’s what (E) gives us.