Font Science
The font and design choices you make for your sales letter influence how your “sales talk,” as Hopkins used to say, is received.
Even if you use almost no formatting at all, that is a design choice.
Mikael Cho writes on the Ooomf Blog about the science behind fonts and how they make readers feel as they make their way across the words and sentences.
https://ooomf.com/blog/the-science-behind-fonts-and-how-they-make-you-feel/