REWARDS - $1000
It seems four people have already planned to rob the store before you and you'll have to chase them down and steal the money from the thieves. Luckily, Catalina shoots one of the men and you'll both hop onto the quad. Your goal here is to tail the other quads and drive close to them as Catalina guns them down.
If you have a drive-by weapon, then you can make this mission go much, much faster. When you finish off one of the cowboys, they'll drop a briefcase that contains the money. You can bypass them now if you want to (they'll appear as green blips on your radar) and just concentrate on the quads. As you progress, they'll split up, so follow whichever one you want. Once the final biker is down, collect all the briefcases and drive Catalina back to her cabin to finish the mission and get $1000.