But the postal system already had immense economy of scale. If Amazon had to bootstrap its own fulfillment from scratch, the per package cost absolutely would h

But the postal system already had immense economy of scale. If Amazon had to bootstrap its own fulfillment from scratch, the per package cost absolutely would have been slashed as volume increased. To put some numbers on it, imagine you start off delivering one package by hand a week, literally by hiring someone to drive across the country. This might cost $2000 a week in fuel, salary, depreciation etc. Now say we're delivering a billion packages a week. The rule quoted would put it at $4 per package. There's wiggle room to argue each end, but considering it's a rule of thumb operating over nine orders of magnitude, that it's not outright nonsensical is actually pretty good. I agree it's not some iron law, and it's important to know when and where it can be applied.