I would think so. Now that someone posted the enlarged version, the glittery red backing matches very well with the outside of the heart. It starts dark on the outside, and gradually becomes lighter toward the middle.
The perspective in both images is different, creating some size ambiguity, but I think it's a match.
I'm just not sure. The actual sticker seems rounder and a little shorter than the backing that's on the envelope. But that could just be my eyes playing tricks on me. But, with the distinct coloration in the sticker, darker going lighter, I would think that a if the backing is somewhere to be found, a definite match could be made.