A computer system is provided for operating a graphical user interface (GUI) to enable a customer to select products. The system comprises a displaying mechanism displaying virtual shelves containing a plurality of first selectable elements in a form of product images arranged on the shelves to represent available products in a group of products selected by the customer, and a plurality of second selectable elements attached to respective first selectable elements. The second selectable elements are in a form of a price tags or price labels attached to the respective product images.