Currently, bonus programs from various companies are very popular, using which companies increase the loyalty of the company's customers, thereby ultimately increasing profits. The article describes the process of developing a web service of the company's loyalty program. The subject area has been studied, different types of loyalty programs have been considered, and its own loyalty program has been developed, which formed the basis for the model of the web service being developed. Next, an overview and analysis of development tools is performed, the most suitable technology stack for implementation is proposed using the example of the .Net Framework platform for implementing the server side and using open libraries to implement the user interface. Based on the analysis of the subject area and the development tools intended for use, a database model of the system was developed, and a database management system represented by MySQL was selected. The architecture of the system was designed using the MVC architectural pattern, Onion design patterns and Repositories, and a scheme for the implementation of the methods of operation of the web service with client accounting systems was proposed. A cashback service project has been developed, which is a web application that interacts with client programs. A module for integration into accounting systems has been developed, using the example of such software products as Topaz-gas station, Frontol and 1C.