I first read this in the late 90's, and it was even then a purple mimeographed copy. My memory is that the speaker offers coffee to the "client" and then they talk for some time about the "client's" intended victim, which the reader discovers near the end is to give time for the poison in the coffee to begin to work. The speaker advises the coffee drinker of his poisoning and tells him part of the cost of the antidote is to leave a written confession of his original intentions, to be published in the event anything happened to the person he was planning to have killed. The very end is something like, "Please, tell your friends about me," with the implication that the speaker is making the world safer by trapping those who would hire killers. Does this ring any bells?
I got no hits from searching Apothecary, Poisoner, or Chemist, all of which seemed faintly remembered or at least plausible titles. I do remember that it's a very short story.
I'll be so grateful if anyone can help me figure this out - such an annoyance to keep remembering more and more details, but come up dry on the title!