Traditionally, product models often have a rigid nature, both with respect to the manner in which they are initially tailored to clients, and to the way they are maintained over time. Especially when such products are offered at a highly interactive medium such as Internet in the form of web services, addressing such aspects might be a necessity to obtain a competitive advantage. To cope with these rigidity problems, this paper proposes a dynamic approach to product models, which supports an ongoing interaction process that continuously adapts a product to the background and desires of the client. The generic approach has been formalised and tested in a case study in the domain of car insurances.