I seem to recall learning in my loshon horah sefer that if someone had ALREADY purchased an item, you are not allowed to go up to him and say I saw it for much cheaper in another store. In this case i think the person was trying to advise you in order to save you money (and I for one would have welcomed that advice). You apparently did not want that advice so you could have politely thanked her and then do what you want. I don’t think it was a great thing to go to the store owner and report to him because it made him feel bad and he could possibly find out who that person is.
From time to time, I give advise to my fellow shoppers, and I ask their advice. especially in a heimish store. We all want to help each other. Prices on food are so expensive I would be very happy to be advised how to save money. Kol Yisrael araivim zeh lezeh.