McDuck's Department Store is named after Scrooge McDuck, Donald Duck's uncle and purportedly the world's richest duck. The store is probably the second largest shop in DisneySea, after Emporio. Like Emporio, McDuck's sells general DisneySea souvenir merchandise: stuffed toys, apparel, candies, etc., all with a DisneySea spin. I think the theming in McDuck's is a little nicer though, with a warm, old-timey department store feel.
Unlike nearly every other department store in Japan, McDuck's department store is only one floor.
If you're looking for DisneySea souvenirs, McDuck's just might fit the bill. (Get it? Fit the "bill"? Sorry, I couldn't resist!)