How much cargo passenger cars carry?
There are no specific answers to this in car owner manual or any other specification documents related to the vehicle. No car maker advertises "our car can carry 50 kg more than other cars in this segment/price range". Cars are meant to transport people. But then people also weigh. A 5 seater means 5* 80 kg per person = 400 kgs in weight. If all are heavy, well built person the passenger weight alone can touch 500 kgs. Then each passenger will have some bags and stuff, so add another 100 kgs. All cars carry 30-50 liter of fuel and a spare tyre weighing 30-50 kgs, so we can factor another 100 kgs. Thus most passenger cars should have an inherent ability to carry about 500-600 kgs of load in them. That apart, car owners occasionally need to move houses, haul some heavy goods around or other heavy duty work. Of course it is easier to say "hire a truck or goods vehicle" but then that comes at extra cost, you've to scout around for a cargo vehicle, coordinate the shipment and so on- at times it is lot cheaper to do it ourselves in own vehicles, particularly when cargo to be shipped is limited and fits well in the boot. Factors that affect a car's load carrying ability  1. Road condition: If it is a direct cruise on well paved highway, cars can handle extra weight as there won't be any abrupt stress on the suspension. If the drive includes bad roads, then one has to be careful- moving cargo, potholes or bad roads affect vehicle suspension. 2. Packaging: If the goods is sharp- like metal or bricks etc- they can damage plastic interiors of the car. Such goods need to be protected well with some soft materials around so that even if they move a bit during transport car's interiors don't get damaged. 3. Weight distribution: If you can distribute the load across the vehicle it may be better, than overloading everything in boot and putting max stress on rear suspension. Again you've to ensure cargo doesn't damage seats or interior elements during the driving  4. Uphill: if your drive involves going up a hill, that puts more load on engine and weight carrying capacity is limited. 5. Engine power: A vehicle with more powerful engine, better suspension can pull more weight. An SUV can handle more cargo than small car  6. Distance: There will be a difference depending on how long you have to carry the cargo. If the distance is small, impact could be less. If you're driving 100s of kms with heavy cargo then risks are more- you may speed up and hit a speed beaker or vehicle suspension fails to sustain non stop pressure of your heavy cargo etc  7. Driving condition: If you have lots of cargo you need to drive very slow. If you don't have the patience and drive like there's no load, there is bigger risk. Because all these factors are hard to predict, car makers never announce max weight carrying capacity. Commercial vehicles however are rated in tons like 2 ton, 10 ton etc, which is also assuming level roads and fair distribution of cargo across the cargo bay. If you have a compulsion to haul heavy cargo in your car, take following precautions