Pay Pal custom Button. ASP.NET C#

I have created a paypal button, that contains a pass threw variable named custom.  My question is, when the user clicks the button, and is sent to paypal check out,  pays, then is sent back to my site. is the custom variable there, or do i have to retrieve it some other way, from paypal?  or do i have to create a cookie to remember the session.  This is all C#
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Andrew AngellCo-Owner / DeveloperCommented:
You would need to setup Payment Data Transfer (PDT) in order for the custom variable to get passed back to your return URL.  

It is not recommended that do any database updates, email notifications, etc. from your return URL, though, because there is no guarantee the user will make it there.  Instant Payment Notification (IPN) is recommended, which will be triggered no matter what.

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