The best kind of ending is the kind that gives you just enough, letting you know what happens and then ending before you hear too much about it. And that's exactly how Al Capone Does My Shirts ends.
We know that Moose manages (or tries) to slip a note to Al Capone, invoking his celebrity power to help get Natalie into her school. And when she gets into the school right afterward, we're smart enough to connect the dots. But how cool is it to read that last little chapter and see that Moose finds a note in the sleeve of his freshly washed shirt?
It's really neat that the author doesn't come out and tell us what happens—she just gives us an obvious tip, which works for dramatic effect. We're sure Piper would appreciate the subtle flair of it all.