## 6 Conclusions, Limitations, and Future Research  
This work presents the use of horizontal cooperation as a way to gain competitiveness in the e-grocery delivery sector. For testing the convenience of using horizontal cooperation, we develop an agent-based simulation model in the city of Pamplona (Spain). We evaluate the effects on the economic and service quality sides of the logistics operations for different scenarios which consist on distribution of online demand orders at supermarkets. Two degrees of horizontal cooperation for performing the deliveries are tested, while distribution plans are determined by the implementation of a biased randomization algorithm. As a result, the use of horizontal cooperation clearly improves the economic and service quality performance of the e-grocery distribution. Furthermore, the distribution becomes more robust to unexpected demand increases when the coalition is able to absorb the new demand more efficiently than the actors independently.  
Nevertheless, there are a number of assumptions made during the modeling process that imply some limitations to our work. Firstly, we assume all the orders contain products available at all supermarkets. Therefore, we are assuming that customers order products that are identical in the different supermarkets in the coalition, such as top brand products and fresh vegetables and fruits. Nonetheless, note that in our simulation experiments, the customers do not change the preferred supermarket, they still order to their preferred supermarket but it is the coalition that internally makes efficient assignments and then the order is supplied, or not, by a different supermarket or, even, by a third party logistic service provider. Secondly, we are considering that supermarkets will accept all orders they receive. That means there are no limitations for dispatching any order at any time window. Thirdly, we are assuming each supermarket owns an homogeneous fleet. This means that all the fleets are composed of vehicles of identical capacity, which does not resemble reality, as many different vehicle types may exist in a flotilla. Finally, we assume there are cooperation agreements as exposed. Actually, horizontal cooperation may adapt many different forms in terms of time frame, amplitude, stamina, and the involved organizational levels [58].  
Regarding future research, two new clear scopes can be considered. On the one hand, there are opportunities to develop more complex simulation models. This involves the access to high resolution data to better estimate demand patterns and supermarket characteristics. Additionally, the ad-hoc definition of horizontal cooperation policies that are currently running on logistics-related services would allow the calibration and validation of forthcoming simulation-based researches. On the other hand, practical issues should be analysed and integrated in the optimization models. This refers to the way in which benefits and risks are shared in the coalition; as well as how the coalition should evolve during time.  
Conceptualization, A.S.-H. and J.F.; methodology, A.S.-H.; writing-original draft preparation, A.S.-H. and R.d.I.T.; writing--review and editing, L.C. and J.F. All authors have read and agreed to the published version of the manuscript.  
This work has been partially supported by the Spanish Ministry of Science, Innovation, and Universities (RED2018-102642-T; PID2019-111100 RB-C22/AEI/10.13039/501100011033) and the SEPIE Erasmus+ Program (2019-I-E501-KA103-062602). We also want to acknowledge the support received from the CAN Foundation in Navarre, Spain (Grant ID 903 100010434 under the agreement LCF/PR/PR15/51100007).  
Not applicable.  
Data Availability StatementData are available upon reasonable request to the corresponding author.  
The authors declare no conflict of interest.