Simply Discount Coupon Field

We need a simple option for discount coupons. I would like to see a textbox where the only validation is to type "mycouponcode" at which point it minuses off x% or x$.

So basically a user that types anything other than "mycouponcode" gets an instant "invalid code". Then if a user types in "mycouponcode" the calculation is applied to the price.

It would of course need to be dynamically updated and validated.