Chez, I agree, and thank you for pointing out that possibility. But in support of HP's theory, it is unlikely that a sticker backing cut out for later use in scrapbooking would be kept in a shoebox with an old letter to Caylee rather than, for example, with other scrapbooking supplies.
It is the location that makes me think this is a "keepsake" cut-out backing rather than a "for later scrapbooking use" cut-out backing.
I agree - I just want confirmation CASEY placed them there and not someone else... a la Cindy the neat-knick. I can so see Cindy rearranging Casey's things (another point of contention/power struggle for them: Cindy: "pick up your room or I'll do it for you!" Casey: "don't you dare touch my things!," etc.)