Welllllllll did this whole story turn into a friggin disaster!
First off these are the "rivets" the friggin estimating guide refers to as "rear body rivets" and the Mercedes dealer was no better telling me that is exactly what they are. Never did figure out what the real rivet price was, I did get some of the cheaper ones Mercedes had, coated pop rivets for 1.90 each.
I don't know where this riv-nut is used. I have to assume it's for a wire loom to stick onto the stud with a plastic clip. But being the car is gone, I don't know.
So the car is gone you ask, well apparently the customer was under the impression that the shop was a "Mercedes Authorized" shop and it's not. So after I spent about an hour and a half checking in all the parts (three page invoice) I am told that the car was leaving and to not open anything, the shop that got the job will be coming by to pick them up!
The car left on the hook in minutes and that was that. I did meet the parts man over at this other shop (A HUGE shop that recently sold to a large chain of shops in the area) and he is a super nice guy, so it wasn't a total waste for me at least. But the shop sure spent a bunch on this mess!