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The search for the most economically advantageous offer is now being made available to public purchasers with a focus on the strategic scope of sustainable purchasing that is seen as creating value for both buyers and suppliers. Nonetheless, the weighting of social and environmental criteria that balance economic criteria requires specific managerial arrangement. As part of the interactive approach of the IMP Group, this article discusses the modalities of mobilization to be considered to favor the choice of better-selling offers. An exploratory case study conducted within La Poste Group shed light on mechanisms that create value for stakeholders but also obstacles to interaction. The analysis of the results leads to proposing a structuring mechanism for the purchasing process with a view to better mobilizing networks of public and private actors towards responsible public purchases.