I can explain it all better to you in the video below, so take a peak and see if you like this idea as much as I do!
The finished products are so professional looking. They look like a purchased gift box rather than a flimsy home made version. The only downfall is the backing boards only come in white and in 8.5 x 11 and 10 x 14 inches, although the larger size is more expensive. I found them listed on Amazon under Magazine Backing Boards. But when you think about it, most envelopes are white, and the boxes can certainly be decorated, embellished or even wrapped. One suggestion: when scoring this type of paper, be sure to score it on the matte side. This will give the glossy side nice smooth edges. If you score it on the glossy side with that intending to be the outside of the box, the edges will be rough and crack. I have attached links to templates for the two card boxes shown in the photos.
Box Template for A2 CardsBox
Template for 5.5" square cards