I think I'm a serial poster!! ..

I was wondering if there is a way to include a radio button for gift vouchers on the checkout_payment page which deducts the full amount of the purchase from users' GV vouchers ... I'm looking to use it with the text box already available so that they can still pay part of the amount with vouchers... but it is easier if users can click a button rather than input a figure to cover the existing purchases..

_thanks