Just found a bug when you are restricting coupons to certain categories using a percentage. The bug causes the redemtion to deduct the coupon amount from the shipping costs.

To fix, open up /includes/modules/order_total/ot_coupon.php

locate line 230

and change

$pr_c = $this->product_price(tep_get_prid($order->products[$i]['id']));

to:

$pr_c = $this->product_price($order->products[$i]['id']);

and thats it

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