mtbc | Slower deliveries; cheaper logistics

An issue I have with Amazon Prime and with Amazon's general assumption that fast shipping is much attractive is that mostly I do not care if a package takes a few days to reach me: I have very few urgent unanticipated needs that are satisfied by a two-day delay. Similarly they will sometimes have dispatched this portion of your order separately to give you a quicker service but typically I would have been content to wait: an I'm in no hurry checkbox on the order page would be one I would often check. Similarly I am occasionally irritated if a postal service confuses a desire for tracked, reliable mail with a desire for speed.

While I am abstractly curious about distribution logistics I am obligated to color my thinking by acknowledging that I concretely know little about supply chain optimization. Questions thus arise in my mind:

Is it reasonable to think that slower shipping in fewer packages does tend to be cheaper for the supplier and kinder to the environment? I can imagine that, for example, sometimes they may be happy to ship a part-order early and transport it faster because it is coming from somewhere else anyway and costs money to store in warehouses and trucks.

Is it any better to be ordering online instead of driving out to the mall? The truck delivering to my home is probably carrying many items at once and using quite an optimal route. Though they do have the difficulty of carrying assorted wares to various places instead of pallets of identical items to my nearest city.