The e-commerce market is projected to grow years by years, including in Indonesia. This growth was triggered by the increasing number of internet users both in the world and in Indonesia. But along with the e-commerce market grow in Indonesia, this was followed by a decline in the number of visits to the retail store. The object is an ad tech company in Indonesia that tries to increase the number of visits to the retail store. The firm uses the mobile application platform to run its business. However, according to a preliminary study, 75% of respondents said they were still having problems when using the application. Therefore we need usability testing for this application to find out how easily the mobile application is used. This study uses the usability testing method with think aloud, after scenario questionnaire (ASQ), system usability scale (SUS), and questionnaire for user interface satisfaction (QUIS). After gathered the usability results from the mobile application, this research was continued by making improvements to the user interface design of the application using the user-centered design approach and using 10 usability heuristics for user interface design and Material Design as literature in the making of usability improvements.