When using PayPal payment services on your Ubercart site, it is very simple to setup your site to receive payment notifications from PayPal. Most everything is handled by Ubercart itself, so all you have to do is go to the Instant Payment Notification Preferences section of your PayPal account profile and turn off the IPN and leave the URL blank. When Ubercart submits a payment request, it will activate the IPN itself with the proper URL.
(For website payments standard, you can also configure the Website Payment Preferences to turn on an auto return with the URL of http://www.example.com/uc_paypal/wps/complete, replacing example.com with your site's URL. This will attempt to force customers back to your website when they complete payment through PayPal.)