Very easy, says the newbie.
Since you have sooo much extra time, how about skipping ahead of your class and reading up on how to write proper OOP code?
If you want to show off some real Java skillz, try rewriting your program to consist of an
InvoiceApp class and an
Invoice class. The Invoice class should have methods to get, set, and calculate values as well as one or two constructors to set common values on instantiation. The InvoiceApp class should generate Invoices based on user input and store these in a list of some sort.