public IHttpActionResult AddCourseToInvoice([FromUri] int courseID, [FromUri] int invoiceID)
        {
            var service = new CourseService();

            if (!service.AddCourseToInvoice(courseID, invoiceID))
            {
                return(InternalServerError());
            }

            return(Ok($"Course successfully billed to Invoice {invoiceID}!"));
        }