This was mentioned in IRC:
Oroborous Lower Sanctuary needs a sending system; even if its just peasant boys "yelling" in the streets.
Since it seems Lower has trouble getting word around quickly, I don't see why a bunch of boys and girls in Lower wouldn't get together and decide to relay messages to major areas in Lower. The price could be lower, perhaps 5gp since the message isn't being instantly sent to everyone at once. The group would listen to what the PC says and then scatter to various locations, repeat the message, and return.
For example, if a character told them "Valerius thinks you're all idiots!" and gave them the money, they'd run off to their locations and say "Valerius thinks you're all idiots!" The character paying for this clearly isn't Valerius...
Or, more constructively, they could deliver a message "Gerard wants a few strong men to help 'clean up' a tomb. Find him in the Crone."
After giving the message, they gather back where they were awaiting another person who wants to send a message. If they can't find a path to their destination from where they're located, they could just run to the nearest door and then jump to their post, and then run to a point in the distance and jump back to the door they "left" from.
Suggested locations to deliver the message: The Crone, the House of Light, the Lower marketplace, the other marketplace. Perhaps other locations that I don't know about, too.