Even acting in good faith, the nature of their shopping experience (goods are undifferentiated, often unpackaged, often either very large or very small and self-checkout predominates) makes it easy to make mistakes. I'll willingly admit that I've gotten to my car and realized I didn't scan (or under-counted) an item at least once (and I'm rather compulsive about checking my work).

You want to save money by making customers do the checkout labor for you, you're going to lose some money on goods.