I don't think you're able to buy things in a conventional way, as in entering your credit card credentials on the website. The payment site accepts only Japanese credit cards and there's no way you can bypass this requirement.
I was told that you can, however, use a proxy server. It seems that non-Japanese paying players on the forums employ this method to purchase items from the charge money shop. I can't help you with this since I'm not all that literate when it comes to stuff like this, but I just want you to be aware that this is one way you can navigate around the restriction of the official site.