alt Hacker News> I suspect delivery of single items cuts back significantly on trips to the store.
Amazon is also specifically incentivized to be efficient at scale, it impacts their bottom line to the point where they care about the shape of their vehicles. Individuals don't operate on the same scale so these sort of micro-optimizations don't happen.
Humans are specifically incentivized to be efficient at scale. They'll tend to shop at loctions on the way home from work, or otherwise cut down on travel times because traffic sucks.
I honestly can imagine that Costco is overall more efficient than Amazon, especially for people who do shop at Costco. If there's no Costco closeby, its more likely that the individual humans will shop elsewhere or somewhere more convenient.