I was zeroing out the amount purchased: The comparison is the customer picks up one item vs. Amazon delivers one item, or the customer picks up 12 or 20 things vs. Amazon delivers the same amount.

I'd still love to see data.

The problem with environmental impact is really a consequence of subsidized energy costs, including the externalization of environmental cost. If the consumer and Amazon paid the actual cost of fuel, they would make valid economic and environmental choices and we wouldn't need to figure it out like this.