in similar case, i must give credit to the way carrefour handle it, once i was charged a different price than the promotional sign, went back to take a photo of the sign, walked to the customer service show them the receipt and the photo, she initially tried to cook up excuse but I'm not convinced, then a young indian chap on roller blade stand next to me asking what's the problem, explained to him, without a blink he instructed the customer service refund in full and gave me the food for free and he skate away before I manage to say anything. Admit that some of their stuff are more expensive but since their conduct is more gentlemen, if it's not exorbitant i still consciously willing to pay a bit more to reward them.
Do Unto Others
"Possessions, outward success, publicity, luxury -- to me, these have always been contemptible. I believe that a simple and unassuming manner of life is best for everyone, best for both the body and the mind." --- Albert Einstein