the best deal on individual units of these things, great. they're not the most rugged, so buy two or be gentle with them.
these work great, the clear snap thing is thin enough to fit into the end of those door opener tools. obviously, it's 50 cents, don't be dumb like me and yank on it, because the string will break, but it's still possible to feed it back through the little knob thing and make it work again, as long as the end of the string doesn't slip into the reel.