Q-commerce has accelerated the growth of O2O retail and increased demand for instant delivery. However, conventional courier-only fulfillment is under growing pressure from rising operating costs and stringent timeliness requirements. This study examines unmanned aerial vehicle (UAV)-assisted instant delivery for an online-to-offline (O2O) retail store with dynamic order arrivals. A mixed-integer linear programming model and a hybrid heuristic combining an adapted Clarke–Wright savings algorithm with discrete particle swarm optimization and adaptive tabu search coordinate courier routes and UAV replenishment under time-window and resource constraints. The collaborative mode reduces mean total operating cost by 4.84%–10.67% relative to the courier-only mode. At lower demand levels, it maintains a 100% order service rate with fewer couriers. When courier resources are fully utilized, it achieves higher order service rates with fewer repeated courier departures. Compared with baselines, the proposed heuristic achieves lower mean total operating cost and CPU time, with equal or higher order service rates. In a case study using 600 orders from a dark store of O2O retailer in Fuzhou, collaboration reduces total operating cost by 20.73% and increases the order service rate from 71.00% to 83.50% under rolling-horizon scheduling. These results indicate that UAV replenishment can improve cost and service performance under constrained courier availability.
