At this time it is not possible to add a fee or give a discount based on the current order total. However, you can offer your customers a coupon (discount) for future orders.
To create the coupon access your control panel. From the Promotions tab go to View Coupon Codes then Create a Coupon Code. At this screen you will have the option to create a coupon based on the order subtotal. You will also have the option to set an expiration date and the maximum number of uses.

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