open your driver (or passenger) door and there will be a sticker that shows maximum safe payload.
GVM is gross vehicle mass. Thats car plus passenger plus sand.
There will be another figure that is lower than that – thats the weight of the car, empty.
the difference is how much you can carry!
But the easy answer is in an A3 – no worries at all, you could manage 5 passengers PLUS a boot load of luggage which would be way over 250kg.
For my job we do silly things lke go to the biggest mountain in spain on the hottest day of the year, fill the car with payload, add a very heavy trailer, and tow it all the way to the top to check everything is still OK. 10 bags of sand would be a breeze in comparison! I dont work for audi but Ive seen them there doing something similar – all half-decent manufacturers do the same