[thaumasiotes]

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.