Choose free gift at checkout

One of my clients wants to do the following.

For purchases where the cart total is over £20 he would like to offer a free gift, the visitor would select the gift in the checkout from a choice of 4 or 5 gifts, probably a radio button with a few descriptive words.

I have searched to see if anyone else is doing this but to no avail.

Any ideas on how to do this or anyone know of a similar mod?
Thanks
Chris