You have to create the front and the back (this is obvious). Add crop marks so the printer knows where to trim the artwork.

A die cut (the cut out rectangle) may be very expensive as this requires the printer to create a die to cut out the shape, and then running the cards through a special press to cut out the rectangle.

What if you put part of the card that is below the cut out, on the card instead of the die cut rectangle. You can add a small drop shadow to give the illusion of a cut out area?