For me, this usually only applies to shaders and emblems since I usually keep 10 on me at all times (including the one I equip) but you should be able to delete items from the postmaster screen. It's a bit irritating at times (especially when most of the emblems are duplicates and/or total shit that I will end up immediately dismantling) that I have to shuffle all my items around to get something from the postmaster only to dismantle it seconds later.
We should be able to just dismantle from that screen.