The Organization for Economic Cooperation and Development decided to investigate the issue and funded a study by 2 engineers, who used the book trade for their case study, analyzing two scenarios: buying a book at a bricks-and-mortar store, or buying it online.

They calculated shipping distances, emissions from individual vehicles, the demands of retail space, airfreight transport vs. local truck delivery and the resources used to return goods.

Their published results found that shopping malls are indeed more expensive and less eco-friendly. The benefit of immediate service is overshadowed by rising costs and the fuel spent when unsold products are returned to the manufacturer. Online shopping tends to have a lower return rate so was easier on the planet.

The biggest carbon footprint is created by all those individual cars driving to the mall. Again, e-commerce is greener in this area, particularly since many transport companies  are moving to electric and hybrid vehicles.  It's even more eoc-friendly if you opt for ground delivery and forego airlifted packages.  Online stores also tend to consume less energy since they store their products in one warehouse not intended for customers. 

In the end, the two engineers, Matthews and Hendrickson concluded, "E-commerce sales have a cost advantage and environmental benefits."

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One drawback to online shopping - no Santa at the mall. (or maybe that's a benefit depending upon your viewpoint)