An Ethical Quandary at the Drugstore

Rena and I were at a mall near Rechovot and we stopped in at the grocery store to buy some vitamins for the kids. As an aside, there are about a zillion brands of kosher vitamins for sale in Israel, in every possible form and size. It's truly amazing. In any case, I don't like to buy new vitamins until we've run out, but our local store stopped carrying our brand, so she walked in to check. When she got to the shelf she took one look at the product which was selling for 47 shekel and said, "Wow. It's way cheaper than where we buy it." (It was at the same chain.) Then we looked at the other boxes of the same product. 67 shekel - the normal price. Which brought us to our quandary.
Is it right to purchase a product when you know definitively that it was mis-tagged? We put it down, picked it up, and then put it down again. Then I had a bright idea: "Maybe the store has a policy that it only charges the price as marked?" Then, the price of the item is whatever the price tag says it is, and not what the store marks the price as?
So I brought the item to the checkout counter. The lady scanned it and immediately noticed the difference in prices. Without either of us saying a word to each other, she marked it down to 47 shekel. I paid and we left the store.
Which brings me to my question: Should I have pointed out the difference in price and paid the extra amount (or not bought the item)? Did I have to do so, or would that just have been the nice thing to do? I'm really not sure about this one.