Roger, I think there's a trend developing here

If it's not too late cancel! I've never heard of this working.
Think psychology. Consider the mindset of people as they use the store.
On the way in, they may be stressed from driving there in traffic, kids screaming, someone cut them up in the car park, trolley wont go the way they point it etc etc... at the point of entering the building they have got tunnel vision.... straight to the first items they want to get. They're not going to stop and look at things like this.
On the way out... they may have got trolley rage, sore feet, backache, kids still screaming, worried about how much they've spent on the shopping, late for the school run, etc etc, you could probably think of loads more emotional turmoil people are going through as they're exiting a supermarket. me personally, I just wanna get the hell out of there and go home.
Peoples' minds aren't in the zone to be marketed to.