1. The CO2 of the Amazon delivery needs to be taken into account (apportioned to the number of drops), vs delivery to store. Delivery to stores is highly efficient compared to last mile.
2. Most Amazon purchases probably wouldn't directly result in less trips to the supermarket (i.e. am I driving to the supermarket just to pick up one item, or am I going there anyway and picking up everything I need when I am there?)
Between that and shopping for fresh meat and produce at ethnic markets that are all over the city, the vast majority of my shopping is amortized over staple groceries that I can’t or won’t buy online.
For use cases like this, the last mile is carbon neutral.
Edit: Re-reading my comment, it comes across as if there were no walkable cities in the US. That was not my intention. Of course there are, but from what I've seen, not that many. Also, somehow there is a lot of political opposition to them, because of certain conspiracy theorists who seem to have taken over one of the political parties. And since there are only two worth mentioning, the situation is unlikely to improve.