Customs agent just takes care of clearing the car through customs, meeting quarrantine, totalling all the various taxes and duties payable for you, and basically signing all the basic paperwork to get it off the dock when it arrives.
You, or anybody can do that - but as you have to be present, most just use a broker to do this work for them for a few hundred and the time.
I assume in that case, that J-spec is doing that for you. Generally there will probably be an extra charge for doing that, unless J-spec is doing this for you as part of their fee. Why not ring them up and ask them?