But then Amazon also uses a bunch of packaging. The cardboard packaging is putatively recyclable, except for the collapse of paper/cardboard recycling in the US last year. I still recycle it, but it's probably being landfilled. The plastic packaging is just landfill waste. And even if the cardboard was being recycled, there's a huge water burden involved in the production of recycled paper products as I imagine it, so trading off carbon emissions versus local water consumption / drought risk seems like a problem.
If someone made a website or app that allowed me to visualize all this for each purchase I make, I think that would be mondo cool -- and as dsalzman's comment to this thread mentions, this could be an opportunity to tie into a carbon offset service or something else to automatically compensate, at least partially, for the efficiency gap.
Come to think of it, that leads to another cost I hadn't considered before: at least theoretically, the money I save on Amazon frees up money for offsets or environmentally friendly donations versus paying more elsewhere. (Although in practice my total donations per year come nowhere close to what I save).
I'd also be curious today whether picking slower shipping on Amazon reduces or increases the carbon burden of the transaction versus prime shipping versus faster shipping.