How to direct buyers to a "thank you" page after Paypal checkout is complete

I have searched the web, and scanned the forums, and have found nothing useful. I use a widget called Shopping Cart And Buy Now (SCABN) to sell products using paypal. I’ve searched the widgets How-tos and FAQ, and have found nothing. I really need help with this, and it would be nice if another SCABN user could give me some pointers.

Anyway, my actual question is: How do I direct the buyer to a Thank You page, where they can download the .zip file that I am offering to them? And only take them there after they have finished checkout through Paypal? And how does this work with SCABN?

Please help, thank you.