I have an item in my cart which is set at zero price. When I add this to cart and checkout, I am still sent to Worldpay to process the payment for £0.00. Is there any way to modify the code so that the "thanks" page is shown immediately if the total amount of the cart is zero?
what module / version are you using to do this?
This may not be the latest version, but the module has been quite customised for this site and is working otherwise, so I would be nervous of making any updates.
You might have to code a fix of your own. Have a look at OrderForm.php, and perhaps introduce a check that the order total is above 0.
I'm not sure of the exact behaviour in later versions of the code, but I can understand if someone wanted to place an order that does total $0. Eg: some kind of 'free' promotion.