One of the decisions I made when I published my book was whether or not to sell a Kindle Version online. I have the file, and can sell it direct without customers having to go through Amazon. The book page on this site has the link to purchase it that way. When someone uses that option, I only have to give Square their percentage, which is very small.
The difference is when someone buys the book on Amazon/Kindle about 60% of the cost goes to Amazon/PayPal/whatever company that distributes to Amazon. So for a book that someone pays $9.99, by the time everyone gets their cut, I get about $4. Luckily, this is an e-version, so it didn’t cost me to print. If I sold the print version online, I’d make practically nothing after printing costs!
Even with self-publishing, there are a lot of pockets to fill with the proceeds of your hard work. If you are thinking of self-publishing, I’d be happy to walk you through the process.