Managecart radio box help

[enigma]

I would like to add something under the shipping row like:

Do you hold a discount card: Yes () No ()

If they select yes then the total price (excluding shipping) is reduced by 5% if they select no then the price stays the same. If they don't select anything an error is prented: please select if you hold a discount card.

They will then enter their discount number on the checkout page.