Here's a tip to save you money. No matter where you shop, check your receipts. Take note as you shop of your 'buy 2 get one free' 30% off or 50% off, two for the price of one and discounted lines at reduced prices.
I have lost count of the number of times I have had to be refunded and it happens in all of the shops, I know it takes a bit of time to check it, but honestly it is worth the hassle. I have written to complain, also I have noted that there is no urgency to rectify the problem by the supermarket. Odd that, as I'm sure if there was a discrepancy of undercharging on an item, it would be a very urgent matter. How many people are over charged daily on these items and don't even notice.
I know everyone is busy these days, but why let them away with it, its your money.