I KNOW someone knows how to specify a return value for product purchases.
When someone finishes his/her purchase, how can i determine what url page they are going to be sent to?

Can this be done for each product?

The page that is returned after the order is accepted is simply:

http://www.yourdomain.com/index.asp?PageAction=CHECKOUTMSG

If you have custom payment methods set up, you can specify what the message says by going to Order Information -> Custom Payment Method, and looking for the Order Confirmation Message. If you are using a real time payment method (such as Authorize.net, VeriSign, LinkPoint, Paypal, etc.), you would go to Order Information -> Online Payment Method to modify it.

You cannot specify a different return link for each product. The message that is displayed is based on the payment that they selected. So you could get it to say one message for when they pay using Authorize.net, and another one for when they pay using Paypal.