Dumplie\Customer\Application\Command\ChangeBillingAddress::cartId PHP Method

    public function cartId() : string
    {
        return $this->cartId;
    }

 /**
  * @param ChangeBillingAddress $command
  * @throws InvalidArgumentException
  */
 public function handle(ChangeBillingAddress $command)
 {
     $billing = new Address($command->name(), $command->street(), $command->postCode(), $command->city(), $command->countryIso2Code());
     $checkout = $this->checkouts->getForCart(new CartId($command->cartId()));
     $checkout->changeBillingAddress($billing);
 }