Swamped

Your best option is to divert them. Get them confused about which direction to run in.

And you do have this book that should be heavy enough to make quite a bit of noise, and, more importantly, isn't actually yours, so they can't prove you were the troublemaker.

You toss it down a side hall as you turn a corner.

"I heard something! Over here!"

You don't have time to breathe a sigh of relief. You just keep going to the Postmaster's door.

You knock very gently. Don't want to be drawing their attention again.

The Postmaster opens the door, and groans.

"Should've known you'd show up sooner or later. This had better be good if it can't wait until morning."

Well. How should you explain this to him?