Not buying ebooks from Amazon.com (from germany) often, maybe once every 3 months, but haven't got a letter from them so far either.
Buying with a fake US account with a semi-fake Alaska address (as in it exists, but I do not live there) and using gift cards from my legit account as money source. And buying from my PC with delivery to Kindle for PC, never from the Kindle directly.
Without using VPN or other stuff, so I have a german IP address. Although I will in the future prolyl only buy there when using spotfulx (which I recommend above flyvpn).