The way it works is users are allowed to have multiple quantities on an ad. if there is more than one quantity when one is sold then the ad still shows and the quantity is reduced by one. when there is only one quantity and that is sold then the ad is expired.
The pricing structure is set for a charge per ad per category price. Any quantity would result in the correct math being done and right new price being charged to the customer.
You would have to alter the code because you want users to sell multiple quantities for the single ad price. Yours is just a different cost concept from what the software is designed to do and what our customers wanted when the quantity feature was added. I doubt most customers running our software would want to charge a flat fee and let users sell as many ads as possible. Your cost concept would mean for instance say you charge 3.99 to post an ad in a category and a user could put in 200 quantity and effectively sell an ad on your site for the next year. That is not the way most people envision it to work. If they charge 3.99 to post an ad and a user wants to sell 10 quantity then they would pay $39.99