If you buy a shoe that's too small, you can return it. If you buy a shoe that's too large, you must buy another pair that fits.  One reason for this rule is that the same shoe size fits different feet differently, so if you buy a shoe that's larger or smaller on your right foot than on your left, it might fit differently on the two shoes. If you buy the wrong size shoe, you may want to send the item back for a refund.