As far as I am concerned, it is fine: there is nothing at all wrong with it.However, some old-fashioned purists insist that less occurs with mass nouns (such as rice, money and happiness) while fewer should be used with count nouns (such as books, coins and apples). And the problem is that items is a count noun. As a result, the purists suggest that it should be "6 items or fewer". In fact, last year the giant UK supermarket chain, Tesco, was forced to change all their express counter signs for exactly this reason. (You can read the BBC World article about it.)
The trouble with this is that more and more people are using less with count nouns. This is what Carter and McCarthy's excellent and thoroughly up-to-date Cambridge Grammar of English: A Comprehensive Guide (CUP, 2006) says about it (p. 772):
Increasingly, in a wide range of spoken and written contexts, less is used with plural countable nouns.And they give as an example:
If there were less than six students in the class, they cancelled it.If this usage is becoming more and more acceptable, why should we worry if supermarkets adopt it? We must accept that language constantly changes, and we need to keep aware of how it is changing.
This is not to suggest that "anything goes". Indeed, there are important distinctions between count and mass nouns, and writers should learn about them so they can make use of the appropriate forms. However, if it is now acceptable for most people to use less with plural count nouns, only pedants will object to it. I think it is outrageous that Tesco was forced to waste money changing what was a perfectly good sign because of outdated notions of what is grammatical.
I suggest Supasave keep their sign. It is admirably clear and (to my mind) perfectly grammatical.
