[Shade7]

Idk what to put in there.
I don't have any car parts at home.

You could go to a local scrapyard/garage and ask for any scraps discs you could possible have. They usually have loads around in their scrap pile. Usually when changing discs mechanics will often put the old discs in the the box where they got the new ones from. Said box could easily of been mistaken for a return and not checked by the supplier upon return. Mix ups of this kind aren’t too rare. So totally plausible for wrong item received method.
Worked in a garage for 3 years btw.

Okay so you think I could just put in my old scraps disc's which I will remove?

Yeah that will work too. Put old discs and pads in and say it arrived like that. It’s quite common actually, we used to get deliveries from supplier with old parts in.