[eluser]nevsie[/eluser]
this might help, but from previous experience with worldpay... it can be annoying to find all the option and settings. i assume your using worldpay junior... in which case make sure you are setting the correct values in the worldpay control panel and not as a variable in your $_POST values.
You can set a dynamic return URL or callback, but you need to do extra setup on the worldpay control panel again to make this work...