“With these stickers, you can append a Wikipedia-style [citation needed] to dubious printed statements encountered in real life.”
This is an excellent idea from xkcd. However, when I tried to order some of these stickers from the xkcd store, I discovered that shipping and handling costs to the UK for these $5 stickers was a massive $15! This seems excessive for what is just a piece of paper, so I ended up not buying any of them. A terrible shame, that.