Thanks Jason,
So after the customer has clicked the checkout button on the shopping cart, they're redirected to the checkout page where they enter their billing information. Then they click 'checkout' again and are redirected to, in our case, the Worldpay website. Does this mean they have to fill in their billing info twice?
Also, If you need to change the Merchant ID you entered in the Checkout Wizard - can you do this without having to go through the set up process again?
Thanks for your patience with my beginners questions!