Every time a parcel leaves in a car that was going anyway, a dedicated journey disappears: no van making the round trip, no relative taking their car just to bring the parcel.
The method, plainly
The calculation fits on one line: delivery distance × average emissions of a car per kilometre. That is the CO₂ of the dedicated car journey that didn't happen.
Which figure to use?
According to ADEME's impactCO2 tool, an average combustion-engine car (diesel) emits about 142 g CO₂e per kilometre, manufacturing included. With this value, a 10 km delivery made on a trip already planned avoids about 1.4 kg of CO₂e. The real figure depends on the vehicle and the source used: it's an order of magnitude, not a measurement.
Where can you see the estimate?
On every mission, every shipment and every delivery tracking page, a small leaf shows an estimate of the CO₂ avoided, calculated with this ADEME value. Voisigeurs find the total for their deliveries in their statistics.
The greenest parcel is the one travelling with someone who was going there anyway.