"I have also a custom code so the as are not published but he would want all ads to be published as soon as the payment is done via paypal (which is the only gateway we have)."

The listing plugin is actually doing that. Perhaps you are reffering to pre-purchasing listings
( http://www.thefactory.ro/joomla-forum/auction-factory/bulk-buying-listing-credits-3527.0.html.html might be usefull - just replace com_bids with com_adsman in the form )
As plain reminders: the listing you pay after posting an ad is only the listing of that ad. Other ads postings will be payed too after posting or listings credits will be consumed if listing has been prepurchased.

Sorin