When you have found an account that you would like to buy, you can apply a coupon to get a discount on the current price. Please keep in mind that this coupon is counted as used the moment that you enter it and that only one coupon can be applied at a time.
To make that happen, just click on Apply a Coupon. 
You will be forwarded to a new page where you can apply a coupon by entering it into the empty field and clicking on Apply Coupon. 
Now you can see the actual percentage decrease and the product price.