I'm guessing it's a combination of their profit over the wholesale prices they get from their suppliers, and a bulk-rate deal from Hong Kong post.
Hong Kong is INSANELY buinsess-friendly.
The U.S. is one of the best business climates in the world, at least as compared to Socialist Europe, or the various third-world kleptocracies.
Yet Hong Kong (and Taiwan) make starting a buisness in the U.S. look like Soviet Russia. John Stossel, the Libertarian reporter from ABC's 20/20 news program did a test. He flew to Hong Kong, got in line at a government office, filled out a form, and rented a space in a shopping mall to sell 20/20 and ABC merchandise. He was "in buisness" within 24 hours.
In the U.S. The process took weeks.
In Europe, months.
In the third-world, forget about it. The amount of bribe money, and the "knowing the right people" factor was enormous.