[mhartl]

Randall is pretty chill about people using xkcd strips as long as you aren't selling them commercially:

https://xkcd.com/license.html

I do offer ebook sales, but of course I'm not selling the individual strips themselves, and the online version is free. Still, your comment has made me realize it's probably a good idea to double-check, so I'll probably drop Randall a line just to make sure.