Matt,
WorldPay do offer a method of integration which does allow all processing to be handled (or appear to be handled) on your site. It's called WorldPay Invisible but unfortunatley is not supported by the current WorldPay module which is available from the downloads section here:
http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_48&products_id=171
The current WorldPay module uses the Select Junior method of integration and passes all of the customer's relevant details to the WorldPay payment pages. The only details that the customer has to complete on the WorldPay site are their credit card number and their security code. The WorldPay payment pages are customiseable to some extent.
The question I have is why do you want to do all the processing yourself? This involves you handling customer credit card details and being responsible for the security of these details. I would rather leave credit card security to the banks whose business it is and who have far deeper pockets than most to handle any litigation that may arise from any security breaches.
Unless he/she is well known and has a good reputation I would think twice about giving my credit card details direct to a merchant over the internet. I would much rather know that my card details are being processed by a financial institution rather than by someone operating a store from their back bedroom. Making the customer aware that their credit card details are being handled via a reputable payment gateway such as PayPal or WorldPay in my opinion is much more likely to give potential customers the confidence to make purchases rather than to put them off.
Just my opinion - would be happy to hear yours.
Regards,
Alan
