The interest in last mile, considered the most important factor in e-commerce logistics, has grown as online sales have increased. To ensure customer satisfaction, e-retailers have focused on providing a complete last-mile service through strategies that include time windows. However, the literature has not yet addressed the impact of this particular service on the last-mile sustainability. The purpose of this article is to provide a comprehensive analysis of how time windows impact on sustainability, through a case study. Results highlight the negative impact time windows has on the economic, environmental, and social pillar of sustainability as they increase the number of kilometres needed to deliver online orders. On the other hand, the positive impact of time windows on the service level is also highlighted.