There is already functionality to do this, I have linked to a gallery showing a couple of screenshots below which will show you where the feature is;
http://postimg.org/gallery/13r5zrfok/278066bb/
All products *should* have this option available, however if you do find a product that doesn't please let us know via our sales / support channel sales@bigorbitcards.co.uk and we'll correct it.