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