This is an example for for PayPal Payments.  Please see all the detailed
documentation on paypal.com about what all the fields mean.  What's most
important here is the "business" email address and the "notify_url" URL.
That's what's needed for IPN.
<P>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="">
<input type="hidden" name="item_name" value="IPN test">
<input type="hidden" name="item_number" value="1">
<input type="hidden" name="amount" value="0.01">
<input type="hidden" name="notify_url" value="http://example.com/paypal/ipn.php">
<input type="hidden" name="return"     value="http://example.com/paypal/paid.php">
<input type="hidden" name="cancel_return" value="http://example.com/paypal/cancel.php">
<input type="hidden" name="currency_code" value="USD">
<input type="image" src="https://www.paypal.com/en_US/i/btn/x-click-but23.gif"
       border="0" name="submit" alt="Buy Now">
<!-- 
<input type="hidden" name="invoice" value="8">
-->

</form>

Notes
<ul>
<li> Paypal wll actually track duplicate invoices for us! (This could
     be considered an information leak, since anyone can see if an
     invoice exists.)
<li> Paypal will re-try (in time-doubling backoffs) until it sees a
     "200" HTTP status from the "notify_url".
</ul>