It's just one of this big red "deposit" mail boxes that are made out of metal and stand there. No smashing, just changing it's...orientation. It's nobody's property but the government's.Would you be having fun replacing your mailbox some morning after a drunk guy smashed it off? I can think of things that would be more fun than that.
Be horrified and think of me as a monster if you like, I can justify it to myself and I can handle any consequences/"what if YOUR mail was in the box?" scenarios. But this is far off topic from the morality of helping someone cheat on homework, so it's the last I'll post about it.