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Yay Courier is a side mission and not required for 100% completion. The mission starts with a call from Cesar: he says that the Yay is leaving San Fierro on a dirtbike (called a Sanchez). Get one yourself so you can keep up with him easily.

When you catch up to him, shoot him off his bike to end the mission. Kill him fast because shooting a hole in his backpack makes the Yay leak out. The reward is the cash for turning the Yay in. You can only do this mission before you complete the Yay Ka-Boom-Boom mission.