Retail products are often experienced through transparent barriers such as shop windows, vending machines or display cabinets. Such surfaces offer opportunities for digital augmentation to enhance the experience at this point of contact. To explore this domain and its challenges, we have developed and evaluated the WaveWindow. This is an interactive see-through display that allows users to interact with digital content that overlays physical items behind a semi-transparent screen. Navigating and selecting content is achieved by waving and knocking on the display. We performed a user study and the resulting user interactions were recorded and analysed, and a number of design recommendations are made for gestural interaction in public settings and their application in a retail setting.