The 7 letters that Shu XinYi sent were no normal letters, well they were, but they weren't. The 7 different cities they were sent to all had mines under the Salvius family name. Shu XinYi had singled out 7 different, relatively insignificant miners with the help of Snow in all those cities. The reason he sent an envoy with the letter was to bribe them for a little inside information, after making sure they are compensated well. The wages of the miners is decided by a city's individual council, obviously the difference of wages between cities should not be a lot.