For the smooth, bureaucratic assassin.
Essentially, it's a way of lacing a sheet of paper with poison. Perhaps just a small bag or bottle of highly poisonous powder that you can apply to just a piece of paper, and the next person who picks it up with bare hands gets poisoned. Or maybe it could be a special stamp that affixes poison to the paper, doing the same thing.
Or maybe even a refillable sleepy pen type thing, where instead of injecting it into others, you write it on paper and the poisonous ink takes effect.
I think one application of the paper poison should only last one pick-up, lest someone make death papers that kill everyone.
The detective could likely detect poison on it, or traces of it if it had already been used.
Maybe on purchase selection one could choose what kind of poison they want, death, sleep, paralyzing, silencing, etc.