The laws of economics are unbreakable. You have to realize that someone is paying for the shipping and that someone is always the customers. Or at least temporarily, investors.
There are other economic principles that apply to shipping specifically. It's a pretty old field.
A last-mile delivery truck goes out full and comes back empty. That's terrible logistics, but the fact that it happens anyway suggests that shipping from your home to a UPS center should be basically free - the marginal cost is nearly 0.
Amazon returns have to go through UPS. If Amazon sends out a lot more stuff through UPS than it receives from UPS, returns might just genuinely be free, occupying what would otherwise be slack in the system.