With this extension you'll be able to set fee or discount to all customer groups available. You may set a fee in percent to one group and discount in fixed value for another one and they'll be applied accordingly. Advanced options allow you to move customers from one group to another and back, when their orders match given criteria.
Sale Page: https://www.opencart.com/index.php?route=marketplace/extension/info&extension_id=23690
Sale Page: https://www.opencart.com/index.php?route=marketplace/extension/info&extension_id=23690