<form id="myForm1" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick"/>
<input type="hidden" name="business" value="yourpaypalemail@gmail.com"/>
<input type="hidden" name="quantity" value="1"/>
<input type="hidden" name="item_name" value="Product Name"/>
<input type="hidden" name="amount" value="19.95"/>
<input type="hidden" name="no_shipping" value="1"/>
<input type="hidden" name="return" value="http://www.yoursite.com/thankyou.html"/>
<input type="hidden" name="cancel_return" value="http://www.yoursite.com"/>
<input type="hidden" name="notify_url" value="http://www.yoursite.com/wp-content/plugins/wp-affiliate-platform/api/ipn_handler.php"/>
<input type="hidden" name="custom" value="" />
<input type="image" src="http://images.paypal.com/images/x-click-but5.gif" border="0" name="submit"/>
</form>
<script type="text/javascript">
var ap_id = getCookie('ap_id');
document.forms['myForm1'].custom.value = ap_id;
</script>