I see that you’ve made the functional console-base billing system, but it lacks user friendly interface. Since I have already worked on this project in my practice session, I strongly recommend you to focusing on GUI next. Consider using the framework like Qt and other option is SFML in C++ to present clear input fields, buttons, and a professional layout. I review your code structure, you can separate your logic from the UI so each part remain more maintainable. It is also good practice to validate inputs. for example, prevent negative prices or quantities. But It is amazing project to improve the coding skillls and make improvements, keep it, I appretiate your efforts.