Simple php shop - add coupon code field

Hi - I have a very straight forward shop template which allows people to purchase products using a shopping basket, once they have finished shopping it goes through a few steps, Delivery, Payment Info etc. - on the Payment Info page I want to add a field for promo/coupon code which deducts 40% off the persons purchase.

I am not entirely sure on the best way for this - the code that is used can be 1 static code (doesn’t have to be removed from the list once it has been used).

If any one could point me in the direction of any tutorials, or has any good ideas on this I would very much appreciate it.