The application of service robots is rapidly spreading across several store types, with robots performing more complex tasks and interacting with customers. A few studies have yet focused on how the introduction of service robots can affect customer responses. In this study, we compare human-human (HHI) vs. human-robot (HRI) in-store interactions across the stages of the selling process and assess how customer immersion in the shopping experience affects store visit duration. Based on a field experiment of 50 participants, we apply a neuroscientific approach collecting and analysing participants' biometrics through wearable sensors during the interactions with a service robot or a human sales assistant. Results show how the presence of a service robot could have positive effects on the shopping experience and visit duration. However, the role of human sales assistants is critical for developing customer engagement and extending the time spent in-store by customers.