No, you do /delivercar ONCE then all the modifications saved will be at the cars spawn location.
No..
You mod the car at the mod shop.. go to a dealer, /savecar, choose 4th option and choose what you want to save (YOU PAY FOR EACH BODY PART).. then you go /delivercar it (unless you haven't done it).. car looses mod, respawn it and it will be spawned with mods.
It's only cause /delivercar does not respawn the car, but basically some-what repairs and changes color on it.. so system only works on spawn

which delivercar does not use.