Does anyone know which add on or piece this is on the rear part of the side skirt near the rear wheels?

 

Veilside addon, there is usually a matching one in front, too.

 

I can't seem to find any on eBay except the fiber glass ones for like 90 bucks and 40$ shippinf

 

They don't seem to be very common anymore. I've got a full set of polyurethane pieces I bought a few years ago and still haven't gotten around to having them painted and installed. I can't even remember what brand they are, but the fit is perfect and I think I paid ~$150 shipped. Not sure about those Chinese-made fiberglass ones on eBay.