On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Neither Rain nor Sleet nor Skags. You can start this quest in the Three Horns Valley location after the completion of the side mission named No Vacancy.
Three Horns Valley
1) Bounty board
2) Bandit board
Approach the Bounty Board  to begin the mission. start off by taking the package from the roof of the bus, reaching it through the roof of the car shown above. From the moment you pick up the package, you will have 1:30 minutes to reach the Bandit camp  and place letter in the designated post boxes. Before picking up the package, place the car nearby the bus to save some time. Driving to the camp, be sure to use boost (by default F) to get there faster .
Some chests are found in the upper parts of the camp, accessible by stairs found somewhere around the building. If you won't make it in time, you will have to return to the bus and take another package.