Add a 'coupon code' option to my paypal paymetn gateway

To sell products on my site I currently use a paypal payment button, which has been altered (via paypal) to integrate with my membership site (wishlist member). I simply take the button code and put it on the website. Simple.

I now want the option for people to put in 'coupon codes' before the buy. So ideally there would be the normal price on screen with the button, and a coupon code box underneath. When they then enter the code they see the price change and then click on the 'buy now' button. The rest of the payment system needs to stay the same and I do not want a complicated e-commerce system at this point.

I hope this is possible and I ideally need it done over the next day.

Thanks,
Dan.