free shipping to retail clients only

file i'd like to change /root/includes/modules/shipping/ freeoptions.php

I would like to restrict all groups from having this option come up in the checkout process, except regular registered customers

if (verify_customer_group_bigspender()) { 
     $content .= '<a class="category-links" href="' . zen_href_link(FILENAME_BIGSPENDER_PROMO) . '">' . CATEGORIES_BOX_HEADING_BIGSPENDER_PROMO. '</a>' .  '<br />' ."\n";
  }

ok i found this piece of code (which only lets certain people view a link in a sidebox) that swguy had created for me a while back the same people in the big spenders group (which are registered members minus all other groups) are the ones i want to offer free shipping to after spending a certain amount

how would i modify that piece of code to work the way i would like?

thank you for any help that can be provided

i found this but i'm still not sure how i could make it work http://www.zen-cart.com/forum/showthread.php?t=76660&highlight=shipping+mod+groups with what i would like