Codified Customs Tarrifs
Contributed by David Verne
One of Rome's most important client states was the small desert city state of Palmyra. It sat on an important trade route, and Palmyra kept the trade routes clear of bandits, who were often the same guards hire to protect the routes, essentially a protection racket. The Empire was happy to let Palmyra remain independent as long as the trade kept flowing, and they didn't extort too much. Eventually, the rulers of Palmyra came up with a better way to make money and decided to set down a system of written customs taxes. This was praised by the caravan merchants since tax agents couldn't arbitrarily fix taxes. The laws were engraved on a stone wall on April 18th 137 and have lasted until modern day. (I couldn't find if it still exists, since ISIS destroyed much of the ruins during their occupation of the site.) 
- Asakura, Hironori (October 2005). World History of The Customs and Tariffs. World Customs Organization. ISBN 9782874920219.