Their prices aren't very competitive (like most of the items on the American Amazon site, as it ends up paying for the free shipping to most domestic customers),
The headmasters have been pretty popular and going up in price soon after release, across the board here. Normally Amazon JP is maybe a few dollars more expensive overall than other domestic stores (most of which charge shipping fee), so Amazon's free domestic shipping is pretty much a farce unless you're buying lots. Aside from that, they recently put in place a 2000Y threshold you have to reach for free domestic shipping too, they say so they can enable the shipping of too small items for free. Which is bullshit, cause they were doing it for years before!