IMHO, LE was taking pictures of all the heart sticker sheets they found and it was before they found the actual sticker at the scene. At first didn't they just find "perfect heart shaped residue" on the duct tape and then the heart sticker itself was found a bit later? I think they were hoping to match something to the duct tape impression, then they found the sticker.
Seems that Casey scrapbooked...I also scrapbook and from the pictures shown of the sticker sheets I don't see one that would match the sticker from the scene. The one that was with the purple envelope definitely doesn't match. "Bubble" stickers typically have rounded edges and USUALLY just come on a clear sheet of backing. It's unusual to find a bubble sticker like that on a sheet of paper backing. In fact I just went through my large collection of scrapbook stickers and I couldn't find any bubble type stickers on paper backing, they are all on sheets of clear thin plastic like backing.