Glad that helps, if you want to see example code where that button is configured see the SetupWorldPay method in the Checkout page for webstore.
Note that what we set as the cartid on the button is not the actual cart guid but the guid of the worldpay log. The cart is serialized as a string and stored in the log and then reconstituted from the log if the transaction is completed. This prevents manipulation of the order. For example if we used the actual cart guid then the user could use 2 browser tabs and after they are posted over to WorldPay they could add items to the cart from the other browser tab and try to get more than they actually purchased at WorldPay. By serializing the current state of the cart just before sending them to WorldPay we can be sure we are re-constituting it just as it was. Each time the cart is modified we create a new log so the guid would not match if the user tries to engineer some manipulation with 2 browser windows. We use a similar technique in all cases where the user completes payment on another site like PayPal, Google, or WorldPay.