Your requirement can be fulfilled by a minor customization in the checkout process provided by default Magento.
Two ways to implement:
1.Store a value into a Magento session and display that value.
2.Create a new extension & store this information with other order informations and display the value.
You need to create a custom module or can install developed extensions in order to fulfill your requirement as direct changes in Magento core file is not a part of standard coding.
Hope this will help you.