Hello Mei,
I would guess the emphasis on your complaint is not not being able to get a replacement screw from another source as Trevor suggests, but the fact that if you choose to get it from Audi, then they force you to buy associated bits which you don't want - and that is a fair point. The hard fact is that car manufacturers supply spare parts and have to stock these parts pending a request from someone. They also have to have an individual part number for every sub part and yes, I agree it would be great if every nut, bolt, washer, screw, fastener was available as a separate item. However, the system to indentify each of these would simply result in it being uneconomical for them to stock the part - so part unavailable!
I applaud your attempts, but I fear you aren't going to get anywhere with this, and you just as well wander down to your local hardware store and buy one, and take satisfaction from supporting a local business.
I am not pro Audi with this - having the same issues with a broken centre arm rest clip on an A3. You've guessed it - you have to buy the arm rest top! Still nice cars though.
Kind regards,
Gareth.