识别谁进行购买PayPal-IPN

Question

I am developing a Twitter web app in which users can buy packages of coins.

Once they click on a paypal button and wants to checkout, they are directred to enter their credentials and finalize the paypal. The PayPal server will then in turn send a HTTP to a backend servlet of mine (Im doing this in Java) with info about the transaction.

My problem here is that how do I know which user made the purchase? The thing is, the IPN message sent by the PayPal server contains a set of predefined parameters from their side. This means that I can not really identify which user made the payment!

Since this is a twitter application, no username/email/password is needed to register for the website.

All I need and want is a way to send the dbUserId of the user that is making the purchase to the paypal IPN request so that once PayPal sends the IPN POST I know who made the purchase and thus add the coins to his/her record.

I need all help please.

Thanks

Answer 1

You can send your ID value in the "custom" parameter of your Standard button code or API requests. This parameter allows you to store anything you want up to 256 characters, and that same value will be returned in IPN in the "custom" parameter as well.

Another option would be to save a local invoice/order record for the user prior to sending them over to PayPal for payment. That way you can send the invoice/order ID in the "invoice" parameter within the PayPal request, and again, that value would be returned in IPN in the "invoice" parameter there.

Either way, you can pull that value and then pull out any additional details you need from your own system within the IPN script.