Justified: Alexi and Bjorn work for rival gangs. Alexi's boss Carlos knew there was a danger of the messenger getting killedanyway, so he sent someone he wanted dead.
"It says you're here to kill me." "Seemed polite to warn you." *BLAM*
"P.S. I've poisoned Alexi without her knowing, as I suspect she might be misdirecting my messages. Please give her this antidote should she deliver this letter in a timely fashion."
Subverted: Bjorn looks up a Alexi and hesitates, his hand hovering over his gun. Then he says "I'd run far away if I were you. Seems your boss wants you dead."
Double Subverted: As she walks out, Bjorn mutters to The Dragon "See she meets with an... accident."
Mercury Couriers demand a Messenger Execution Deposit up front.
The letter ends "P.S., shoot the woman who delivers this", and Bjorn immediately does so. Then he reads further; "P.P.S., change of plan, that woman's been taken care of. This one's my niece, I think you two would really hit it off!"
Zig Zagged: Bjorn reads the letter, and pulls a gun. Alexi, suspecting betrayal, is too quick for him, and they end up in a Mexican Standoff. Alexi kills him and reads the note... but it specifically says "Kindly let my messenger leave unharmed to deliver your reply, old friend." When Alexi reports back to Carlos, he tells her that "old friend" has long been the Friendly Enemies' code phrase for "do the opposite of what I'm asking"...
Averted: There's nothing in the note about Alexi, and she returns with Bjorn's reply.
Alexi slides the letter under the door. "If there's anything in there about killing me, just know I've got a Sawn Off Shotgun pointing at your office door right now. You can send your own reply to Carlos; I'm getting out of this cutthroat business while I can."
Discussed: "Let me guess, it ends 'P.S., kill the messenger'?"
Conversed: "Wouldn't it make more sense to get somebody else to deliver that message?"
Implied: Alexi has recently pissed Carlos off, but doesn't think he knows about it. She's sent to deliver a letter to Bjorn, who the viewers know (but Alexi doesn't) is Carlos' wet work guy, and the last shot of the episode is her entering Bjorn's office and shutting the door behind her. Neither Alexi, nor the contents of that letter, are ever discussed again.