Why does the woman prefer sea transportation?

Man: From what I’ve heard, you’re already well up in shipping work. How do you like the goods dispatched, by railway or by sea?
Woman: By sea, please. Because of the high cost of railway transportation, we prefer sea transportation.
Man: That’s what we think.

Man: Hello, I’d like to place an order for a number of your Millennium desk units.
Woman: Certainly. How many were you interested in ordering for purchase?
Man: 10000. Do you have so many available in the warehouse?
Woman: We only keep 5000 in stock. There’s also a showroom with 6000 on hand. It shouldn’t be a problem.