In this era, buying and selling transactions can be done through e-Commerce platforms. Because of the high demand, various types of e-Commerce companies began to appear. Some of the leading e-Commerce companies in Indonesia are Shopee, Tokopedia, and Bukalapak. The process of purchasing goods is essential in e-Commerce, making the system design of the process as a major concern. Therefore, this study aims to improve the usability of goods purchasing activity flow on the mobile version of the three leading e-Commerce platforms. Usability evaluation was done using mixed methods by combining quantitative and qualitative data. The quantitative data were obtained using online questionnaires, while the qualitative data were obtained using usability tests by mapping the results to Schneiderman's Eight Golden Rules of the Interface Design. Online questionnaires were distributed to validate and to find problems that commonly occur in the process of purchasing goods. Usability testing was done to find the problems more deeply. From the overall study, 13 usability problems were found and an alternative design was devised to solve them and to improve the usability. Finally, the alternative design had also been evaluated with a 83-100% success rate for each task.