I have started stringing the parts on wires so they can put them on hooks that hang down from an assembly type line that goes through the plating tank, and that gives me a more uniform coating, and it also gets in the recesses that barrel plating misses. I started doing it this way because my plating shop stopped using barrels, but they still charge the same price. It does take longer to wire the parts, but I think it is worth it.
It sounds like you may not know about the acid dip they do before plating, so you shouldn't get spring steel plated, the acid will make it brittle (Hydrogen embrittlement), and of course it will eat up aluminum. I was able to talk them into skipping the acid dip once, just depends on how cooperative they want to be because this is not a money maker for them.