Save Cart in Hikashop Cart Class

Hi,

You're mixing the cart object which contains only the cart data loaded in the PHP memory and the cart class which allows you to access the methods to interact with carts in the database.
You should write:

$cartClass = hikashop_get('class.cart');
$cart = $cartClass->loadFullCart($cartId);
$cart->cart_coupon = '199';
$cartClass->save($cart);