I see the decal idea as similar to a "certificate of participation" when you help out at a charity or something - of no intrinsic value in itself, but nice to be recognised by the people you're helping.
I once did a charity motorbike rides thing for "Camp Quality", a mob that help improve the life of kids with cancer, and they gave me a Camp Quality tshirt. That was nice and I wear it with pride, but I did it to help, not for the tshirt. I've got a similar tshirt from an old people's home in the Philippines where to volunteered to be a photographer for an event they were holding... and a stuffed toy dragon from an orphanage in Vietnam called "Blue Dragon" that helps out street kids. I've also got a bunch of tshirts, badges, patches etc from various charity rides, christmas gift runs, and so on I've been on over the years. I'm proud of all these things because they remind me of times I've helped, but they're not why I helped. They help me remember how good it felt to help, they encourage me to help more, and they make me feel like my help was appreciated. They can also encourage others to help out in future - not for the tshirt or the decal etc, but simply by raising awareness.
A couple of pics from various charity runs for the Cord Blood Bank at a hospital near me.