Need Identifier to link payment to an advertisement

How do I link a payment of an advertisement to an order?
We have thousands of advertising’s and each can be payed with the shortcode like:
[wp_paypal button=”buynow” name=”‘.$ad_id.'” currency=”EUR” amount=”12.00″ target=”_blank”] means name = field item_name

In this case I have linked the field name to my internal table “wp_ads” with the var $ad_id.
But how do I get the field data “item_name” from the table post?