code: public function setCartReference($cart_reference)
      {
          if (!is_null($cart_reference) && (strlen($cart_reference) > 200)) {
              throw new \InvalidArgumentException('invalid length for $cart_reference when calling CheckoutOrder., must be smaller than or equal to 200.');
          }
  
          $this->container['cart_reference'] = $cart_reference;
  
          return $this;
      }
docs: Sets cart_reference
  @param string $cart_reference The shopping cart reference id
  @return $this