Install of interior kit - rip off !
It all should come down to labor when you supply the parts. It probably takes an experienced person professional 2 to 3 hours say to install a headliner how much should it be charging hourly, 50-60 bucks? $1500 for all seems bit steep and 4200 I'd run from fast. These guys just sound pissed because you bought the stuff and they can't make money on their mark ups!
I had the same problems trying to get my car painted, I got qoutes for 10 grand, my cars not worth that much! I stripped the car and did the body work all they had to do is prep and paint it. Body shops seems to make to much on collision work to bother to earn their keep these days!