javascript表单： - 如何为总计提供不同的变量值

hey guys so basically this form is meant to have each chapter of the book at £2 which i have and then the entire book-PDF will be £8 and the entire book printed will be £15 plus postage and packaging... i currently can not seem to get the last two prices and postage to differ from the chapters would appreciate any help thanks hey guys so basically this form is meant to hav