Digital Downloads As Additional Option To Prints

Is it possible to add digital download as option  to photo product?

When I am selling a print, I'd like to have two options: 

1. to sell only a print, without the possibility of downloading a digital,

2. To sell a print with the digital as a product option - so the client can download  the digital  of this print, but ONLY if they choose this option (it will be extra paid).

I can't figure it out how to allow downloading digitals only when "print + digital" product option is chosen, and not allow download if the "print only" option is chosen.

See: Offering Download Photos With Purchase Of Prints