plastic painting is different than body painting.. have u ever wonder why does the bumper that is repainted tend to peel off compare to stock from factory?? most of the aftermarket painter is not really specialized in plastic painting.. Plastic at its raw form will not be able to be painted as paint coat does not have any adhesion to the plastic.. hence specialized primer coating is required to introduce adhesion surface for the paint coat.. Normally the painter outside does not apply the primer.. maybe they want to save cost or maybe they dont even know the thing is required..
therefore, my advise is seek help from reputable painter.. they should be able to give u a much better answer in term of cost and stuff.. i'll update again if i have additional info..
By the way, the surface u r tryin to paint is PVC or PP??