As an English teacher, this is an English grammar mistake that bothers me when I see it. However, it is very common!
The grammar rule is about modifying a countable noun versus modifying an uncountable noun.
She has less money; she has fewer dollars- these are correct. “Money” is uncountable and “dollars” is countable.
Another example: there is less coffee in my cup. You wouldn’t say: there is fewer coffee in my cup.
In the comic above, the word “items” is countable, so the sign should say “fewer items” and not “less items”.
But as I said, this mistake is very common with native English speakers. Keep your eyes open and I’m sure that you will start to see this mistake often.