## 1 Introduction  
During the last decade, consumers' shopping habits have drastically changed, not only because of the massive incorporation of new technologies into our lives but also because of a greater awareness of environmental and social sustainability, growing urbanization, and time pressures. The outcome is a challenging scenario mainly driven by the increment on demand for e-groceries (i.e., the online purchase of groceries, including fresh products) because of an exceptional development of e-commerce. As a consequence of this paradigm shift in consumption, companies have also adapted to this new order by adopting proactive sustainable strategies and developing sustainable supply chain management practices. However, and despite the complexity that features the growing demand, the existing literature does not demonstrate the challenges of the field, especially those related to the logistic process. Further, despite the promising development of the e-grocery business, the lack of interest in developing cost-effective operations is also evident in companies, since there are only a few e-grocers which have made progress on leading to profitable operations [1, 2]. Thus, the challenges in e-grocery logistics include from a wide-range of food safety-related issues to differences in storage temperatures, including perishability over time [3]. In addition, environmentally responsible customer profiles must also be considered, who demand consumption of local products [4]. Note that, considering these consumer requirements, it is more than likely that consumers' wishes differ from the seller's desires. While consumers usually prefer products from a close origin with long date of expiry [5], sellers would benefit from shipping first items with shorter shelf lives in order to reduce food waste [6]. In fact, many researchers recognize the strategic importance of sustainability as an essential aspect in the supply chain management [7,8]. It is widely accepted that sustainability cannot be achieved by companies in isolation, and that requires the involvement of supply chain members [9]. Reinforcing the same idea, [PERSON] and [PERSON] [10] plead that supply chain members operate in more dynamic environments, characterized by globalization, rapidly evolving technologies, and increased customer responsiveness. Therefore, more integrative and cooperative efforts are required to reach the aforementioned supply chain characteristics. Likewise, horizontal cooperation may be paramount when meeting the requirements of demand and sellers in an efficient and sustainable way, for example, improving efficiency in logistics. Therefore, the partnering sellers aim at increasing productivity through close cooperation, for example, by optimizing vehicle capacity utilization, reducing empty mileage and cutting costs of non-core/supporting activities to increase the competitiveness of their logistics networks [11,12]. At this manner, [PERSON] et al. [13] have enumerated the potential benefits of cooperation as follows: _(i)_ reduction of cost of transportation; _(ii)_ improvement of service quality by reducing operation times and lost goods; _(iii)_ diminution of environmental and social impacts; _(iv)_ mitigation of risks; _(v)_ and enhancement of market share. Consequently, extrapolating the previous benefits, horizontal cooperation might be particularly interesting for e-grocery, where a wide range of customers are widespread in big cities or in rural areas, which generates long empty backhauls after deliveries. In this regard, load factors can be improved by means of cooperation (i.e., supermarkets sharing their logistics operations) to reduce empty backhauls.