Opencart Discount logic in checkout section

Hi,
I am new to OpenCart. I have mad a small site with OpenCart. The reference link is http://gmentr.com/msplocart/index.php?route=common/home. Now you can see when you will go to the product description page there is the product discount section.In that block of when I am entering any one value for example just take 2 which discount price is €135,00. Now I entered 2 in the input box and it is redirected to the checkout page. Now you can see in the checkout page quantity is 2 and price is €135,00 but in total it is showing €270,00.It is actually taking logic as per product price is €135,00 and multiplying with product amount. But here I want the discounted price shown in the product desccription page with the amount of product will be same the total price in checkout page. So can some one tell me how to put this logic in Opencart?