Note: You are welcome to reprint occasional comics pretty much anywhere (presentations, papers, blogs with ads, etc). If you’re not outright merchandizing, you’re probably fine. Just be sure to attribute the comic to xkcd.com. Can we print xkcd in our magazine/newspaper/other publication?
If it’s a not-for-profit publication, you need no permission — just print them with attribution to xkcd.com. You can post xkcd in your blog (whether ad-supported or not) with no need to get my permission.