Less vs Fewer

As a former English teacher, I try not to get to picky about other people's grammar because I know that I still make plenty of mistakes as I write and speak (of course, I always told my students that I taught writing, not speech and that we typically do not speak the way we write--hopefully).  However, there are certain words that I often see misused that stand out to me because I taught specific lessons on those words.

One set of words is "less" and "fewer."  Typically, "fewer" should be used when people want to refer to items that they can count like chairs, speed, typos, and money.  People should use "less" when they want to refer to items that cannot be counted like courage, brains, and chances.

For some reason, the other day I realized that many stores with express lanes use the wrong word on their signs.  They often write "12 items or less" when it should actually be "12 items or fewer."  I guess that explains why some stores avoid this problem all together by simply writing "About 20 items."