Consider the code we reviewed for Friday's class, and especially the use of the Receipt class. Think about how it was initialized and how it was used. Then write an interface specification for it. As you saw during lecture, the interface, since it can be implemented by multiple classes, should have a name different than the class.
As an alternative, you may write a class specification for the Receipt class, but leave the body of each method specification empty. Remember that a class specification is, provides an implicit definition of its own interface.