Adding a cost for extra pictures

Hi all,
I have my site almost completed after 5+ months. My site allows my registered users to post items up for sale on my site for free. What I want to do is create a way that my users can pay for extra pictures of their products. ie.. A user can post two pictures for free but, if they want any more then that a charge of .50 cents of something like that, they will need to pay.

I am using Drupal 6, Ubercart 2, and Ubercart Marketplace.

If you have any thought it will be helpful.

James