They ripped me off again!

I was full of cold last week and didn’t check my grocery receipt that carefully. Looking at it this morning I realized that they had charged me twice for an item. At twice in three weeks that’s a 66% error rate – hopefully this is just a blip as I like shopping at our local Pak n Save. I’ll have to see next week if they believe me as I don’t really have any proof – how do you prove that they scanned something twice weeks later?