Simple contribution to xt-commerce website. "how did you hear about us" vote after checkout_success.php

We want a functionality where customers can via check-box, chooes where they found our website/hear about us. The function must come after they place order in checkout_success.php. WE want to be able to quite easily change the possible answers, not neccesarily in admin interface, but atleast only in 1 php/txt file. The webshop is XT-Commerce based. (a derivative of oscommerce).