The cart sends the "Success URL" parameter as defined in the PayPal docs using the following code:
url = String.Format("http://{0}:{1}{2}", Request["SERVER_NAME"], Request["SERVER_PORT"], EditUrl("Confirm"));
PayPal.SuccessUrl = String.Format("{0}{1}", url, "?PayPal=Success&rm=2");
So the cart sends the SuccessURL (Return URL) when the user is redirected to PayPal. In addition, you need to upgrade to a business account at Paypal and set the return url to your site "http://www.yandasmpa.com/"
Did you upgrade to the paypal business account?
Can you post what you entered as your site "Return URL" in your paypal settings. Did you include the http in front of the domain?
Also, you mentioned in your post that the cart was sending the return url to "Storefront/tabid/99/ctl/ProductDetail..." can you post the full url that the cart is sending to paypal?
-Scott