Traditional cars used to be built with body on frame. Chrysler was one of the first to go with a unibody (unitized chassis). Technically there is a sub frame, but the firewall, floorpan, rocker panels, etc make the 'frame' of most Mopars (and most all cars built today). Anyhow, that is why you are getting the frame off comments.
'Frame Off' can be used as a slang for body disassembly, although I do not believe they would disassemble the body and paint for $15,000. I'd ask more questions about what you get for $15,000 as far as metal work and prep, paint materials, etc.
Although you did not ask for my advice, I'll offer it anyway. You have not had the car long, and generally speaking paint and body should be one of the last big bucks you send on a car. I'd get to know the car a bit better, see what else it needs mechanically, see how reliable and satisfied you are over some time, then think cosmetics. In the meantime, research some shops. Believe it, or not, many dealership body shops have guys that take on side work. This could be good, or it could be 'paint jail' (you won't see your car finished for many years). Ask around at cars shows who has performed the paint jobs, and get information.
Take your time on the paint job work!