Are people just buying more, or are they moving from buying in stores to buying online?

I don't immediately see how the latter would be bad for CO2. Online shopping is like public transport for goods. Instead of having 100 people drive their 100 cars downtown to go shopping, a delivery truck drives down the street and hands out packets. Returns are obviously more than when you buy in a store where you can try it on first, but how much impact does it make?

The last mile is loads more efficient with online shopping, but all the miles before that are worse -- rather than stores buying densely packed pallets you have a monumental amount of additional packaging being added, and the items are optimized in trucks as best they can be given upwards of 90% empty space in their boxes. Since truck fuel economy is only weakly dependent on weight this winds up using a ton more energy.

I don't have hard numbers though, which makes it hard to answer questions like "do the last-mile efficiencies outweigh other inefficiencies?"

I think it could go either way. I usually don't go to the store to buy only one thing, I do my weekly shopping or at least put together a list. With free shipping and Prime on Amazon, I don't have to think. Just order my Alexa to buy it and it arrives tomorrow. Its even easier than putting it on a shopping list.

Not to mention, where I live in Europe its actually easier to take public transportation to the store most of the time.

The basic problem is that the advanced Western economies are finally starting to reduce their CO2 production (although not very fast), but that the rest of world is not.

I'll believe that when the average Brazilian, Russian or Indian produces more CO2 than the average American, Australian or Brit.