Secret Garden by Graphic45 came out a few years ago. I loved it so much that I bought 2 stacks fearing that it would disappear. And it did for a while. I just wish it was available in digital form so I wouldn't have to feel like I have to "hoard" my stash. Luckily, Graphic45 re-released the product again. PHEW! I found it at Scrapbook.com.
The Everyday Cards SVG Kit from SVGCuts has the perfect "shadowbox" card. BONUS! It folds flat for mailing. I chose to up-end the card so it was vertical and not horizontal. I added some flowers and leaves for a little more dimension. The flowers were made using some glossy paper from some packaging. The flower centers were also cut using the glossy paper and colored using markers. The leaves were cut using white paper and colored with Tombow markers.
Because the print on the back side of the box cut was too busy, I cut a rectangle insert using the same side as the outside of the box; and then added the focal piece cut from one of the sheets in the stack, on top using some pop dots.
You can also insert a piece of transparency to the "window" if desired and transfer a sentiment. I left this one blank so the recipient can have a little more tactile experience with the card. The flowers, leaves and flower centers are also cut using the Sizzix eclips2 and eCal2.