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?