If you enable featured ads on your ClassiPress site, your customer will see the option in the ad posting form in the form of a checkbox where they can pay for the featured listing (or not), depending on your payment settings. You don't need to approve them manually unless you have chosen to moderate ads. What happens when the ad expires is dependent on your settings. It will either be sent back to draft mode and be unpublished from the site, or it will remain on the site with "this ad has expired" on it. No, there is no limit to the number of featured ads posted. Thanks.