I stopped at a Moto M&S a while ago, and noticed this sign. It's a strange form of signage-only punctuation that is becoming nearly as prevalent as the "grocers' apostrophe".
Placing key parts of a message to customers in inverted commas troubles me not so much because it's grammatically bizarre, but because of the tone it creates.
The inverted commas destroy the meaning of the message, making it formulaic or perhaps even sarcastic.
Tiny details like this are important to get right if you want to create the right feeling for a customer. If the concept of the "total customer experience" has any value it is in the word total. Every little thing matters in creating the right feel.
As if to prove that M&S are capable of getting this delicate choice of language very right, I noticed when I went to buy a sandwich the other day that they have a checkout especially for customers with "5 items or fewer". Genius.
All the supermarkets, with "5 items or less", are grammatically wrong. Ultimately what matters most is whether their customers feel that the communication is natural and of the right tone. "5 items or less" is probably the right choice for most of them.