Gift certificate does not offset Shipping cost

I am using Virtuemart 2.10 and Awo Pro. When I test on my site, I found the giftcerficate only offset the product cost not the shipping. Such as $100 total in my shopping cart include $70 products and $30 shipping, if I apply $100 gift card to this order, it only reduce $70 dollars and still asking $30 shipping from customer. I don't like this set up and it really does not make good sense to the shoppers. Did I miss some configurations? How can I allow the gift card apply to the total bill amount rather than product?

Hello,

When you created the template used in the gift certificate product, did you select a function type of 'coupon' or 'gift certificate'?

Hi I really appreciate your reply. I did use gift certificate. I followed the exactly the steps as the documentation. Could that because I use a component called one page check out rather than standard virtuemart checkout? Is that possible to let me know which file calculate the actual discount at the checkout? Or I can PM you my site's admin login?

Yes, might be easier to look at it myself, if you can send the access. and the file that calculates the discount is
www/administrator/components/com_awocoupon/helpers/estore/estorecouponhandler.php