Companies are increasingly held responsible for the sustainability of their own operations as well as their suppliers’ operations. This study aims to investigate the effects of 3PL provider sustainability in triadic relationships. Specifically, the authors study the effects of 3PL provider sustainability and delivery costs on end customer satisfaction with the selling company. In addition, the mediating role of customer-company congruence is examined. Building on logistics and marketing literature, the authors develop a conceptual model, which they empirically test by means of a scenario-based experiment among 192 end customers. The findings support the hypothesized model: 3PL provider sustainability is positively related to end customer satisfaction with the selling company, whereas delivery costs are negatively related to end customer satisfaction with the seller. The effect of 3PL provider sustainability on end customer satisfaction with the seller is partially mediated by customer-company congruence. The results suggest that companies outsourcing their logistics services to 3PL providers should, in addition to costs, use sustainability as a selection criterion and performance indicator in their relationships with 3PL providers.