E-commerce web sites that sell highly customized products tend to be very complex in terms of the underlying logical model of the customization process. We present a systematic approach to guide the design process of such complex web sites by providing a clear methodology that reduces errors and assists the web architect. We also describe our application that helps web architects go from the definition of the product's attributes to the generation of the finished web application that sells the product. The approach is presented in terms of a case study involving the development of a web site that sells customizable T-shirts.