...how does that decrease the volume of shipments?
It doesn't decrease the volume of shipments, but most delivery trucks aren't 100% full. So they could make fewer trips if customers were willing to tolerate delivery delays.
Do you have numbers on that? In a city like NYC I find it hard to believe that there's many delivery trucks with a suboptimal load. With the scale they're dealing with there's no reason that most trucks can't be 100% full, except for poorly written software.
And Amazon is also promoting the concept of a "Delivery Day", presumably to encourage batching deliveries.