At what shipment size does sea freight beat air express?

As a rough planning rule, air express stops being economical somewhere around 100–200 kg or about 2 cubic metres, where sea LCL starts to win on total landed cost. Below that, the fixed costs of sea freight — export documentation, port handling, customs brokerage at both ends — make the per-kilo rate irrelevant, and a small parcel sent by air express is cheaper door to door despite a much higher rate per kilo. Above roughly 15 cubic metres, a 20ft full container load usually beats LCL, because LCL is charged on volume with a minimum and the consolidation handling adds both cost and handling damage risk. The crossover is genuinely lane-specific and moves with fuel, capacity and season, so it has to be recomputed per shipment rather than assumed — but the direction of the rule is stable enough to plan against.