Even the weight out a bit you'll be OK. Put some on the back seats in bags, obviously. As they're designed to support 3 adults.
Those empty one ton bags are good for moving stuff too. Put your back seats down, cover on those, then one of those bags on that. I've moved plenty of stuff in the past using those going to skips.
Three hundred kg is the maximum limit. In these 300 kgs are also included the weight of the passengers. I went on a trip last year, and it was a total catastrophe. I went shopping and also found a lot of great decorative rocks that were heavy, so on the way back home, the car was too heavy, and I destroyed the car's gear box. An Opel Astra can carry around three hundred kilograms without damaging the gearbox. Since then, whenever I go on a trip I weigh all my luggage by using the weight scale I bought from vont.com