You need to explain to your cat that it's a federal offense to tamper with the mail!
Perhaps your neighbors need a more secure mail box.
Cats can have goofy ideas about what's fun. We discovered that one of mine chewed a large hole in a vinyl shower curtain last year. She will nip off shoe laces if anyone leaves shoes out of the closet. She likes to play fetch, and the perfect toy for that is a plastic hair curler. After she's finished for the day she'll knock it under a piece of furniture. When the TV service repair person pushed the armoire aside recently to get at the wires, I was a little awkward explaining why there were 20 or so hair curlers among the dust bunnies.