PayPal IPN works differently, PayPall sends a “Post” request after your transaction to a IPN script on your website, if PayPay can’t get to your server you will never get a notification.
Also you should not be using or getting “Get” request to get back to the site that is the link that you are getting, the plugin for extra security only accepts post request.
What is your PayPal service? and from what country and currency are you testing?