If you don't know the psychology of retail stores, then you enter into every retail experience--both online and brick and mortar--at a disadvantage. That's because the psychology of shopping has been extensively studied--if you don't understand the pressures you face, you're bringing a knife to a gunfight. The Atlantic's Derek Thompson outlined 11 tricks that stores use to encourage you to make a purchase. I'm especially partial to #5:
Savvy restaurants, for example, design their menus to draw our eyes to the most profitable items by things as simple as pictures and boxes. Good rule of thumb: If you see a course on the menu that's highlighted, boxed, illustrated, or paired with a really expensive item, it's probably a high-margin product that the restaurant hopes you'll see and consider.
That's right. Outback doesn't peddle their giant, fried, 2000 calorie onion because it's tasty; they push it because fried onions and special sauce are cheap. This is the type of information that should be taught in high schools.