Fu Yurou planned to take advantage of passing by to drop the letter straight in and leave.
That way, it would be untraceable, and no one would know she was the one who reported it; whatever happened afterward would have nothing to do with her. This way, she wouldn't face retaliation while still doing a good deed. After all, a wise person does not stand under a dangerous wall. She was just a timid young woman who only wanted to survive quietly, without getting mixed up in major affairs like spies.
However, plans never keep pace with changes.
Chapter 118