It's real easy! If I can do it you can do it! And man did it take me a long time to finally try. You just Google Amazon Japan, it comes right up. You'll probably want English, which is offered right there when it comes up. You sign up like you would for anything else. Name, password, and you'll probably want a credit card if you plan to buy something ;) That's really about it... I'm sorry I didn't look at this sooner. I thought it was going to be some big confusing thing.
Placed my Zenkaiger order on the 19th, and it arrived today, the 22nd. It was $74ish after shipping. Some sellers might not ship outside of Japan, but seems like many things do. They handle customs and that fun stuff, there was no additional charge that we've found yet. The only complaint is when I tried to sign in again, they sent me a text to my phone. I probably had to say "yes" but it was in Japanese! I picked the top button (there were two). So... it's the top one, if that happens to you. Otherwise, Amazon Japan seems like a good thing to know.
Let us know if you're still having trouble, but do try first, it's honestly pretty easy, and I'm bad at things like this.