PayPal IPN
PayPal IPN (Instant Payment Notification) System for loop-back to your website's Back-Office:
"Instant Payment Notification (IPN) is PayPal's message service that sends a notification when a transaction is affected. Once IPN is integrated, sellers can automate their back office so they don’t have to wait for payments to come in to trigger order fulfilment."
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There are also pre-built scripts once you're set up if you google for them.