In the scientific literature, there are numerous studies with different approaches and focuses on assessing the environmental impact of online shopping and shopping in the traditional retail channel. The aim of this work is to analyse scientific studies that quantitatively assess the environmental impact of transport activities in both channels and to extract the factors used for this assessment. A literature search was conducted for the period 2006 to October 2020, with 90 studies shortlisted, of which 15 studies were identified as relevant in a screening process. The analysis showed that a different number of factors is included in the selected studies. Logistics-related and behavioural factors are mostly of similar importance. Third-order effects, such as rebound or complementary effects, are rarely considered. Furthermore, it becomes clear that the results also depend on differences in study design and external factors. This work illustrates the complexity of quantitatively assessing the environmental impact of online and in-store shopping. Caution is advised when deriving recommendations for action from general statements about the environmental friendliness of a distribution channel. The 15 factors found, together with the classification method used, form a solid basis for building new models.