IPN works like this. On your site, user enters credit card info and click submit, your code calls PayPal API to submit the payment info. Paypal receives your request. PayPal creates a transaction. (1) It calls the IPN url on your site with transaction info. You can process the info. In your code, (2) the API returns the transaction info. You can process the info. I don't know if IPN callback is made if the user clicks the PayPal button to go to PayPal to make the payment.
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