While here in Okinawa( or anywhere in Japan for that matter), it is often convenient to order things online from Amazon, but using the US site is not so convenient (long time to ship, high shipping fees or not shipping overseas at all). So the answer is Amazon.co.jp.
You need to make a new account as your overseas Amazon account will not work (you cannot just switch countries). No problem. Some things will be in Japanese, but a lot of the shopping screens and menus can be shown in English (switch the language setting in the account to English, shown in the video), so it is actually quite easy! Below is video that shows how to set up an account and order your first item (pickup instructions for Family Mart below!)
For delivery options, you can have it sent to your address in Japan (sometimes you can even set the delivery date/time ahead, otherwise if they deliver when you aren’t at home, they leave a slip with a QR code, you will then need to scan QR code on the slip for re-delivery. You can then pick the day (including the same day! and a 2 hour window for delivery), but it is also possible from the start to have it sent to the nearest convenience store (such as Lawson or FamilyMart). This is the best option if you live on base or otherwise don’t have a regular Japanese address. Even if you have a Japanese address for delivery, sending it to the convince store means you don’t have to be home to sign (In Japan you almost always have to sign or stamp, rarely (like 15% of the time) the will leave a package) You can not only pick up at the conbini, but you can also pay for it there if you don’t have a credit card or otherwise run into trouble verifying it. Talk about convenience. When your package arrives you will get an email with some codes to put into the kiosk at the store. The email will have a link to instructions, including screen shots of the kiosk so it’s not so hard to do. In the worst case, print out your email (or bring your phone) and show it to the clerk, they will help you on the machine. Don’t feel too bad, I’ve seen many Japanese people get assistance from the clerks as well!
As for payments, foreign credit cards are accepted (for everything except for Digital music). Video, regular items, books, and everything else except for Digital music can use foreign card! Otherwise, you can also utilize cash on delivery (to your house or at the convenience store). Yes, this works in Japan, I have done this before. It sort of amazes me. The drivers even carry change if you have it sent to your house or apartment. There is generally a small COD fee, around ¥1-200 per order.
As a student, I qualify for Student Prime membership, and it is only ¥1900 per year. There is also regular Prime membership for only ¥3900 per year, unlike the US which is ~10$ a month. You can access Prime video, Prime shipping, and special deals. The Amazon “brand” videos are worldwide, so you’ll be able to watch things like “The Expanse”; however, many videos are still region locked. To use Prime Music though you must have a Japanese credit card . So keep this in mind, as this is the only frustrating thing for me.
Amazon Video has many movies and TV shows in English (with Japanese subtitles). If your Japanese is good enough, you can watch the regular Japanese movies and TV as well (but then again, you probably are not reading this if your Japanese is so great). It has just about everything the US Amazon Video has, plus some. Titles in foreign languages generally only have the option to subtitle in Japanese not English, so you may miss out on some things. But (As of 2018) it does have all of the old season of X-Files, so that’s pretty awesome.
Oh, and I should mention… you earn points that turn directly into cash discounts when you order from Amazon.jp. Often you can get bonus points, just for signing up or certain items will have extra points attached. Really, it is a pretty great service, and when I realized how cheap the Student Prime membership was through Japan, it has definitely been worth it. I have purchased books, a camera, kimono accessories, and even liquor from Amazon.jp… often times it is cheaper and there is more variety than buying from the department stores.
Update Late 2020: Please leave a comment with questions or let us know how these instructions work! Our goal is to help out folks in Okinawa and Japan, so we are happy to make notes and change the video if someone has some trouble!