finish the inventory unit test
finish cart findCartItem() uint test
finish cart findCartItem() uint test
finish addCartItem() unit test
finish addCartItem() unit test
refactor getTotalAmount() with _reduce
refactor inventory toString() unit test
refactor getCartItemText() & getPromotionText() unit test
refactor getSaveAmount() by _reduce
finish getTotalAmount() & getSaveAmount() uint test
finish getSummaryText() uint text