It'd be interesting to see how many people actually use that option to consolidate a significant amount of items on Amazon, though.
There's also an unstated major premise that packages never come from any further away than the nearest Amazon distribution facility. I suspect that the truth is rather far from that. It at least isn't true for me - I kept an eye on my Prime orders from last year, and 1/3 - 1/2 of the things I ordered originated from a more distant Amazon warehouse, and getting it to me within 2 days involved burning a whole mess of jet fuel. I suspect that the GHG emissions from doing that end up being a massive portion of Amazon's overall pollution.
May not have a huge impact on your overall point, but bump that number up to maybe 20 miles and I actually would bet that the majority of Americans at least do live that close to an Amazon distribution center
https://en.wikipedia.org/wiki/List_of_Amazon_locations#Fulfi...
I suppose that if an occasional item ships from across the country, then it would be less impactful to go by truck for 5 days, so if that is the case then any item that falls under that category could have a selection item on the checkout screen that says "Delay shipment by 3 days, to save 3 lbs of greenhouse gasses". That may influence consumer behavior.
BTW, what is the greenhouse gas emissions difference between one delivery vehicle making the rounds, vs several hundred people each driving to a store to buy something?