Normally for each coupon code, it's written how much percent discount you will get.
To know exactly your discount amount first, add your desired products to the Shopping Cart and go to Check Out page - http://www.shadmart.com/index.php?route=checkout/cart.
On the first 1 step of checkout you will find the section for applying coupon codes.
Click on “ Use Coupon Code” . You can see a section which says “ Enter your coupon here”. Enter the coupon code & simply press on “ Apply your coupon code “ .
Once you successfully enter a correct coupon code. You will see this message.
And on the last step of checkout, you will be able to see the discounted amount.