Magento – Add Coupon Code in Last Step of OneStep Checkout

We are looking for a clean, easy, close to core solution to add coupon code in last step of onestep checkout

Question: What's the best extension or code could we could use to accomplish this

What it should do is

1. show the discount box in the very last step
2. on enter or on submit, it should reload the last step in onestep checkout (na ajax necessary)
3. On the server compute the discount & show the last step again in onestep checkout

many thanks in advance

FYI
When a client goes directly to checkout (instead of via cart) – they are never confronted with a box to enter a coupon code. We have just launched a national ad – and the below article is true … it is almost ridiculous how many emails we receive on this matter: "Hi, we would like to buy but cannot find where to enter the coupon code"

http://www.commandc.com/2014/01/29/top-reasons-for-cart-abandonment-2-difficulty-with-promo-codes/