You can use real car paint but you'll need to use a suitable primer. I use Duplicolor primers exclusively. You'll want to wash and sand the body before the primer. Can't wait to see what you do with that Supra.
I've been know to contact sellers about what I feel is excessive postage. Sometimes they agree to lower and sometimes they don't. If they do, I bid. Otherwise, not.
Just saw the kit you posted. Mine was looking pretty decent until the tape mishap. Most of the paint came off with a dunk in brake fluid. Cleaned the body off and stuck it back in for more removal.
I glued the interior into the body shell of the 427 Cobra I'm building. Used a piece of styrene rod as a shim. Slowly removing the tape today and it peeled some paint off the side of the body. I said some nasty words. Body & hood are soaking in brake fluid right now. Maybe tomorrow I get to do the antique copper paint job all over again.