You are not asked to implement any kind of user interface. We have not even studied standard keyboard input to Java programs yet. The test program will run and use its own coded data examples to send standard output to the screen to enable us to check that everything worked properly. The question is to see whether you can design a bunch of classes to work together in a sensible way. It might be a nice idea to have Magazine class which contains the arrays of supplements and customers.
Some students go to great lengths to avoid passing complicated objects into methods or constructors.
For example, when constructing a customer it is far more sensible to pass in a supplement object rather than just a String for its name. Give an UML diagram and do use the right symbols to distinguish inheritance from composition. Remember that a diagram gives a very clear general overview of design. You will really appreciate diagrams when you have to debug or maintain other peoples software. And you will lose marks if you avoid them or make them less clear by inventing your own notation.