I don't know how great your Grandmother will find it, but I find your comment about calibre Web being difficult.. it is certainly no more difficult than buying a book from Kobo web site. Just a couple tricks to make your life easier. Use the mobile version of Calibre web, (full version won't work at all.) and put your username/password in the URL, / bookmark. So, if your server was example.noip.org on port 8080, type in the following url in the kobo web browser and bookmark it (add to favorites, optionally, set as home page):
http://myusername:mypassword@example...rg:8080/mobile