Someone at Trader Joe's had the good sense to correct their sign. Bravo.
"Less vs. Fewer" is one of the few grammatical pet peeves I have. You want to end your sentence in a preposition? Go right ahead (although that's not technically a rule). You like to dangle your participle? ... Knock yourself out! Don't know the difference between "Affect" and "Effect" - and you're too lazy to Google it? Me too!!
But the confusion between "Less" and "Fewer" makes me crazy. This is a pet peeve I inherited from my Mom ... so blame Mama Bunny if you don't like it.
It's all about "Count Nouns vs. Mass Nouns". LEARN IT.