Although it has been studied extensively throughout the last twenty years, the environmental impact of e-commerce can still be considered a controversial subject. Particularly for those wondering whether online shopping constitutes a more environmentally-friendly alternative to traditional store-based shopping, evidence can be found that quantitatively supports affirmative as well as opposing claims. Findings differ widely because the contexts and assumptions of the studies from which they are drawn, differ widely as well. To advance our understanding of this question and inform actions that can actually reduce the environmental impact of shopping, we carried out a systematic review and meta-analysis of environmental impact assessments that compare the carbon footprint of online and store purchases. Based on over twenty scientific studies, we compiled a dataset of almost 250 purchases, their estimated carbon footprint and information on the contextual, transport, behavioral and geographical conditions on which the calculations are based. We conclude from the studies that online purchases generally generate a lower carbon footprint than store purchases, but only when car-dependent lifestyles are assumed involved; and possibly only because overall consumer behavior and transformations in the consumption landscape are largely overlooked in the studies.