If I’m seling something for $15 and it costs me $3 to ship it, I could bundle the costs together into $18 + free shipping. And people will prefer free shipping even though they’re the same price because our brains interpret the first as two losses and the second as one loss + a bonus cool thing. Monkey brain doesn’t care about exact numbers, only gains vs losses

And that’s cool, but what if you buy two or three things from me? Now you’re technically paying $9 in shipping, not that you’re aware of it, but my shipping costs have only gone up to like $4. You just got screwed by $5 and I get to pocket that extra money