@MiniTaurus I think this old
guide should still work. I'm not sure about which credit cards will work specifically, but my Visa's seem to work the most reliably. I use my US address for billing, but use a Japanese address for my country setting, as the guide tells you to do.
Be sure to check if your card is just trying to prevent fraud, rather than it just not working on Amazon.jp. Anytime I use a new card or replace my old one, my purchases get denied because of fraud protection until I call my credit card provider.
Right now a lot of the manga on sale are 330 yen, which is about $3 USD at the moment. And I definitely understand that purchasing power varies by where you live/your situation, so I don't think you should worry too much if you can't afford to support all the creators that you would like to. It's great that you're trying, though!
If you need me to translate any error messages or something, feel free to PM me.