Ive done affiliate marketing fulltime for a few years, back in the day but not for amazon. US companies would require you to submit W-8BEN form. As far as I remember they've never charged me anything.

I would suggest you worry about all this only when you start making money.

Treat it as a biz, and declare based on your income & refer to sole proprietorship's tax brackets. If you're not making anything... dont sweat it and focus on making your first sale.

Malaysia if you're operating in Malaysia.