Combine that with people's reluctance to use public transportation, and you have a CO2 nightmare.
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?
I don't have hard numbers though, which makes it hard to answer questions like "do the last-mile efficiencies outweigh other inefficiencies?"
Not to mention, where I live in Europe its actually easier to take public transportation to the store most of the time.