option for signature on delivery

Apologies in advance: this should by easy to fix, but as I don’t know the correct terminology I am having trouble simply Googling the answer.
I am using table rates, and currently the customer can choose between Regular and Express postage. All works well. I’d like to be able to offer the customer the choice to also have optional “Signature on Delivery” at a fixed cost, regardless of the postage rate. I envisage that it would be basically a radio button that the customer can toggle on or off. How do I go about this?