From what have seen in the project title and the file you attached, it is a python desktop app that uses tkinter for the interface and should calculate the bills of customers, it has a function for clearing the screen and evaluating the operations but some other stuffs are kind of confusing, more inline comments as to how your functions work will really be helpful, but generally its a cool project and am still looking into the code and running it, would also like to contribute to the project.
Look forward to hearing from you.