Update May 2011 – This may be against terms and conditions. See Martin Brown’s comment below
It took a while to get my head around it, so here it is incase I need it again.
Integrating with WorldPay is not the simplest of tasks. The documentation is poor at best.
I was integrating by using a form on my site, which posts the order total and an order id etc to WorldPay. WorldPay then deal with taking the money from the user. What I couldn’t understand was how to get the user back to my site for thanks after they had paid.
Turns out it’s quite simple.