Hi Aaron,
The return URL I indicated in the PayPal preferences is the root URL of my website (and Smith Cart seems to be happy as it displays the 'Thank You! Your order is confirmed.' page).
But it seems that SmithCart already indicates to PayPal the return URL when sending the payment request (the URL sent to PayPal contains the 'return' and 'cancel_return' parameter), that's how PayPal knows where to redirect the user after after he clicks on the return link.
So I think you must only turn on the auto return URL in the PayPal preferences. See https://www.paypal.com/cgi-bin/webscr... for how to configure it.
But as I stated before, the return to your website is done after few seconds: "With Auto Return on, instead of having to click a button on the payment confirmation page, payers see an alternative payment confirmation page for a few seconds before PayPal returns them automatically to your website."
So the user can still close the browser window before returning to your site.