I have to admit I would ring the store and explain what happened, and yes I would pay. My conscience would make me! I wouldn't be able to look at the item again without feeling shitty.
This kind of thing has happened to me twice! One time I walked off with 2 lots of loo rolls from a local spar, that were in the bottom of the pram, and when I rang them they told me to keep them. And the other time was when me and DH went for a meal at a little restaurant with the kids. Should have been about £70 in total, and for some reason the transaction came up as £4. For some reason the waitress had done something wrong that resulted in us only being charged for the 4 cokes we had when we came in! I put in the PIN at the end and never even noticed what the amount was - silly me, coulda been £300!!!
Anyhoo, I saw the receipt when we got back to the hotel, and saw it was £4 that we had been charged!
We knew it wasn't right and it was a sweet little place with very few people in the Scottish Highlands. So we went back the next day and said we have been massively undercharged. They said they DID notice the £60+ discrepancy and wondered where it had gone wrong, and they were very grateful and took the other £66 off us.
Next day, the waitress saw us in tesco and came up to us and said thank you, and it was her who made the boo-boo, and if we had not come back, she may have lost her job, as that was the 3rd boo-boo she had made that month.
So that is worth thinking about. if we hadn't been honest and gone back, a woman could have lost her job.
Do the right thing; ring them and tell them you want to come in and pay what you owe.