A grandma might be pushing it, but my SO learned how to use Calibre and she isn't very tech-savvy.

Also, Amazon isn't as easy as it sounds - I used it a few times and found it quite confusing to get it on my Kindle after purchasing on Amazon. And its a nightmare as I moved country and would prefer to purchase a larger selection of ebooks in English from my home country etc. but I have a separate Amazon account and stuff.. that was even more confusing.