I too was having problems with Zen Cart not receiving orders from WorldPal, and callback error 302 when transactions were made. WorldPay wasn't sending customers back to my site either.
The solution I found was to add this to my root .htaccess file:
php_value session.use_only_cookies 0
I've not yet had a live order (it's a new shop - www.furnitureonthemove.co.uk/shop), but cancelled orders (which I have made) and test orders all performed correctly within WorldPay and Zen Cart.
I hope this helps!




