Fleet money buys delivery minutes only near the congestion knee: while couriers are near saturation each one added strips several minutes off the average delivery, and once utilization falls below about 80% the next courier removes well under a minute yet still costs a full day's wage.

Daily cost is a straight line in fleet size; average delivery time is a hyperbola in it. The entire tradeoff lives in the gap between those two shapes, and it collapses into a single elbow on the cost-versus-time curve.