This. I’m not sure if you’re able to purchase books for the Kindle app via Amazon,jp without a Japanese address at least on file. It doesn’t have to be your main shipping/account address, though. I know at least this because I had trouble downloading free stuff on my Kindle Fire despite living in Japan and having a Japanese address on file (I had a US address as the main due to using a US credit card). I had no problem ordering books and having them sent to my device however. (Hopefully that makes sense).