On the other hand, if I'm going to drive to the store, I'm likely to plan ahead a little and pick up a number of items in one trip, rather than ordering them one by one online. (Or even ordering them all at once through Amazon, but having them arrive in a dozen separate deliveries.)

I don't remember when I last drove to a store to get a single item. (I do occasionally walk or cycle down to the high street for an individual item such as milk, though it's more usually a handful of things.)

Read a piece recently on how Uber is primarily used by people who are a) fairly well off and b) have access to transit or other options to get to work

This argument you make reminds me of that. Most people taking advantage of Amazon One-Day delivery are in urban centres, and most of them are likely doing OK, and most of them can likely walk or take public transit to get what they need

Except that's the last mile, and I think a small proportion of the total carbon footprint.

I think the question of whether same-day delivery is good or bad boils down to whether the goods are able to be trucked across the country in an efficient manner. If the company has switched to many more smaller trucks, which may be less than completely full, then it's a negative.

Has anyone seen data on this?

I recently starting making an effort to purchase things like paper towels, Advil, toothpaste, etc from Target (in person) instead of buying from Amazon to reduce the amount of packaging I have to recycle and throw away.

I still feel like it's a net benefit, but that's purely a _feeling_.