yes, can be done, local post office around here operates western union, up to 10,000 pesos per day per person and i think up to 50,000 pesos per month per person, exchange rate at the official rate, but i'm told if you go to the main office on ave. santa fe? in capital you have the option to get greenbacks or pesos. Some limits might be different as they update from time to time(not long ago the limit was 7,000 pesos per day per person)......to my knowledge fees apply in the US from 4% to 8% depending if you wire money from your phone or you go in person to the western union office in The States.
Good luck Pilgrim