^
It is far, far cheaper to ship from an on base address to California than from a Japanese address to California.
They have flat-rate USPS boxes at the post office here on base. Sending a letter to anywhere in the USA is just one stamp, like it is in the USA.
Sometimes for large items or for certain retailers, they won't mail to the US bases overseas (the "State" for pacific bases is "AP"). If that happens, you can use a mail forwarding service like
www.apobox.com