I recommend this approach over any other options because there is less chance for breakage. It’s independent of the
customer’s action. If the customer closes the browser or navigates away, you will still receive notifications from PayPal at your
notify_url. IPN also has built-in retry mechanism. If there’s a problem reaching your notify_url, PayPal will re-try for several days. With
you only have one shot at getting the payment data.
<form method=”post” name=”form” action=”https://www.paypal.com/cgi-bin/webscr?”&gt;
<input type=”hidden” name=”rm” value=”2″>
<input type=”hidden” name=”cmd” value=”_xclick”>
<input type=”hidden” name=”business” value=”paypalemail@targetdomain.com”&gt;
<input type=”hidden” name=”item_name” value=”Shopping Cart Order”>
<input type=”hidden” name=”quantity” value=”1″>
<input type=”hidden” name=”notify_url” value=”http://www.ReturnTargetURL.com/index.php?cmd=notify”&gt;
<input type=”hidden” name=”cancel_return” value=”http://www.ReturnTargetURL.com/shop/index.php?cmd=cancel”&gt;
<input type=”hidden” name=”return” value=”http://www.ReturnTargetURL.com/shop/index.php?cmd=thanks”&gt;
<input type=”hidden” name=”invoice” value=”inv_12-2007_8be5b”>
<input type=”hidden” name=”amount” value=”14.95″>
<input type=”hidden” name=”currency_code” value=”USD”>
<input type=”hidden” name=”custom” value=”TokenName||o987y8——SESSION—ID—HERE—-o987y8||0″>
</form>
Heres a link to a real good community forum

Instant Payment Notification (IPN)
How does it work?
After finishing the payment on PayPal, the customer is auto-redirected to your page (”return” variable)
Customer returns to your page. PayPal does NOT send any payment data there.
Separately in the background, you receive a form POST from PayPal at a different URL (notify_url variable).
You post back a form with cmd=_notify-validate and all fields you received from PayPal. PayPal responds with a single word VERIFIED or INVALID
If you receive VERIFIED, you can be confident that the form you received came from PayPal and wasn’t tampered with. Do whatever you need to do with the form fields.
Settings:
Specify an auto return url in your profile or in the return variable in your html form. The url must be an absolute url. This is just a generic page with no PayPal processing logic. Display something like “Thank you and your order will be processed shortly.”
Specify an IPN url in your profile or in the notify_url variable in your html form. This is where you process payment data from PayPal. The IPN url must be an absolute url. It must also allow anonymous access from outside of your network. If you must open your firewall to a specific host, please note the Sandbox sends IPNs from ipn.sandbox.paypal.com. PayPal live site sends IPNs from notify.paypal.com.
Auto Return = Enabled in account profile
PDT = Disabled in account profile
IPN = Enabled in account profile