You're question is a little loaded since we can't see details of the paint work repair estimate and the cost savings by proving your own paint from a Auto Paint Supplier is marginal. For example, does the price quote include different levels of prep work (sanding using 240, 340, 400, 500 grade paper), rust removal, priming, door jams, any auto body repairs, is the paint 1 or 2 stage (i.e. one solid color or based coat then clear coat), no signs of overspray or sandpaper scratch marks guaranteed, quality of paint, warranty period, etc.
Look at some of the cars the vendor has performed completed work and compare the difference with other vendors. Although there are shoddy repair shops that charge a lot, generally speaking, you get what you pay for.
$1,500 for a quality paint job for the entire car is not outrageous. Trying to save a few bucks providing your own paint will probably tick the vendor off.