Paypal NVP with IPN for confirmation - what ties them together

Question

I am using PayPal with NVP API (using PHP) for express checkout. I am creating an invoice record in the database before redirecting the user to Paypal. In case the user doesn't return to my site after processing, I am using IPN to confirm the purchase and then update the invoice record that the payment is confirmed. I am still in the sandbox mode and trying to figure out how I will tie the transaction started with NVP to the confirmation I get with IPN.

I need to verify if the "PAYMENTREQUEST_n_INVNUM" sent in the NVP will come back as "invoice" in the IPN post.

It appears I cannot actually test this until I am live since the Sandbox IPN does not seem to be active with NVP initiated sandbox transactions - is this correct?

Thanks for your help.

Answer 1

You can test this in Sandbox. But if you're using "PayPal NVP", I assume you're using PayPal Express Checkout and calling the SetExpressCheckout and DoExpressCheckoutPayment API's.
If that's the case, you don't really need IPN, because a transaction will only be completed as soon as you call DoExpressCheckoutPayment.

In other words, buyers will always be redirected to the RETURNURL you specified in SetExpressCheckout, and the transaction is completed (or not) when you call DoExpressCheckoutPayment on this return page.

To get the invoice number, you could call GetExpressCheckoutDetails and supply the TOKEN you retrieved earlier (it's also appended to the GET of the RETURNURL).

Finally, check PAYMENTSTATUS=Completed in the DoExpressCheckoutPayment API response to see whether the transaction has completed or not.

Answer 2

Thank you Robert for the clarity on the process - especially useraction=commit.

I finally realized that I could turn on IPN in the Sandbox for my test seller and test NVP with IPN together. I was able to verify that PAYMENTREQUEST_0_INVNUM matches the 'INVOICE' parameter in the IPN POST.

I will use the custom field to pass customer email from my system in case they use a different email to log into paypal with, therefore allowing me to have email/invoice number pair for confirmation.