You could hand a letter to some scruffy street urchin to deliver, but how do you know it hasn't been read by prying eyes elsewhere before getting to its intended recipient? How do they know it hasn't been modified along the way? Why, you'd need some kind of tamper proof seal that would make it immediately obvious if anyone but a master rogue had broken into it!
This item listing is for the wax seal stamp. To purchase the wax to use with the stamp, see here: Sealing Wax Sticks