Yay Courier is a side mission that becomes available after CJ gets the San Fierro garage save location, it is not required for 100% completion. The mission starts every wednesday and saturday in game with a call from Cesar: he says that the yay (drugs) is leaving San Fierro on a cross country bike (a Sanchez dirtbike). Try to get one of these cross country bikes yourself so you can pursue and keep up with the courier more easily.
When you catch up to him, try to line up your bike directly behind the couriers bike, and then shoot him off his bike, you will need a micro SMG for this. Alternatively you can pursue him in a car, and attempt to ram him off the bike, however this can be hard to do as the bike is very agile and the courier is difficult to knock off. After you have shot at him, you need to kill him fast because shooting at him puts a hole in his backpack and makes the Yay leak out. The more that leaks out, the less cash you will recieve at the end of the mission. after you have killed the courier, collect his backpack to end the mission. The reward is up to $2000 in cash. You can only attempt this mission before you complete the Yay Ka-Boom-Boom mission.