Why did she carry the box around?

It seems overly inconvenient to carry around a huge box like this.

Why didn't she just take the glass key and the instructions in her pocket? It seemed small enough to carry around.
And it wouldn't arise suspicion, especially when you're about the go to the mayor's office to expose him.

But even in the end, she kept carrying that huge box.

Maybe she thought it served another purpose...

Thoughts?
Was it like that in the book?