From the Super Mario Wiki
Carson is the bartender for The Overthere, the bar in Flopside. He appears only in Super Paper Mario. For ten coins, Carson will tell one of approximately fifteen stories he has collected over the years. They range in scope from the Pixl uprising to Count Bleck's minions. If the player does not have enough coins, has listened to all of Carson's stories or doesn't accept the 10 coin charge, he will tell one the player has already heard before for free.
His Flipside counterpart is named Garson, and while Carson sells coffee, Garson only sells milk. Carson and Garson take the roles of Russ T. and Wonky from the past two Paper Mario games. The original Paper Mario's Russ. T offered much more information throughout the game and would tell it for free, while in the sequel, Wonky had a wider range of topics and would tell them for five coins.