Specifying the IPN Notification URL
Merchants that have chosen to configure PayPal as a payment method must set up the Instant Payment Notification (IPN) on PayPal's system. After logging into your PayPal account, click the Profile button and select Profile and Settings.
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Next click on My Selling Tools located on the left hand side of the screen as shown below.
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Now click Update for the Instant Payment Notification section as shown below.
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Click the "Choose IPN Settings" button as shown below.
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Finally enter the URL https://secure.ultracart.com/cgi-bin/UCPayPalNotify into the Notification URL field, select Receive IPN messages, and then click Save as shown below.