The simplest reason is that I made a mistake.
I was sent to buy milk which was on sale. I bought the wrong milk. It was on sale, but not the right milk or the right sale, so it cost too much. I was sent back to return it. Here begins the system.
For health and marketing reasons, the milk will be thrown out rather than resold. It is perishable, so it could have gone bad or at least been left out long enough to go bad sooner than could normally be expected. This is a small health risk, but a more significant marketing risk. Customers want a reliable product and will go somewhere with trustworthy milk.
I could return the milk because allowing customers to return items helps maintain their business. To make such an unprofitable concession builds goodwill, as much as can be expected toward a chain. It is expected, having become fairly standard, so to not allow returns would drive away customers. It shows some faith in products, that the store is willing to risk the wasted time or profits, since they believe their products will not often be returned, and if there are defective products, the customer doesn't bear the full burden of that. They let me return the milk because that is less costly than not having me as a customer.
I then walked back to the dairy section and picked up two gallons of milk of a different brand. What a strange scenario, where I turn a small profit of a couple dollars, for effectively destroying a worthwhile product. I'm in the defense industry or a record company, so this is unexpected. It seems more rational to instead give me the difference in price and let me keep the current milk. I get my money and they get to keep the milk. Everyone benefits. For now.
It's the reverse of the return scenario. When returning a product, the store loses in the short term of the one transaction to safeguard future profitable transactions. In this case, the store would gain, but it would be backing a simple scam: buy expensive perishable, return to buy less expensive, take difference and keep more expensive. For a person with sufficiently cheap time, or who likes evening walks, this would be a way to get expensive items more cheaply. It could also be an accounting and inventory issue for the store.
In the future I will pay better attention when buying milk.
But I will mourn the fact that the most economically sensible fix for my mistake was blocked by fear of the dishonest. Then again, when all parties are acting out of economic self-interest, it would take a lot of naivete to think that the other person will not scam, cheat, and steal the first chance they get. Maybe this has something to do with self-destructive self-interest.
But I'll still demand the exact brand to buy next time.
Wasteland: Savage Village
1 day ago