The Outer Worlds: Spacer's Choice Edition

The achievement unlocks when you deliver Olivia’s journal to the dead drop location on Byzantium and conclude the dialog with the contact standing nearby. Although this is simple enough, there are some complex preconditions beforehand.

Following the story, once you use the terminal in the CHEM Lab and Override Synthesis & Manufactory Center Lockdown, you complete the Experiment Perilous main mission and start the main quest Red Tape of Command. This is a branch point. To keep all your options open, make a hard save at this time and do not overwrite it.

Note: If you are going for the best ending in the full game, you will want to complete the game having unlocked the Family Reunion Peril on Gorgon questline ending. The full game's ending is heavily influenced by how you concluded the Gorgon questline. The Postman questline ending is one of the bad Gorgon endings, perhaps the worst. Accordingly, my recommendation is to do it first and, in the process, unlock this achievement (and Greenlight Protocol). Afterwards, load up the insurance save and knock out the remaining achievements.

With the insurance save in hand, this is the roadmap to the Postman Peril on Gorgon ending and this achievement:

1. Go to the Ambrose manor and kill Minnie
2. On your ship, ADA patches you through to Lucien. Agree to get him Olivia’s journal.
3. Go to the SMC, get the journal, and return to your ship
4. ADA patches you through to the interested parties, concluding The Red Tape of Command
5. The Ambrose Intersection main mission is automatically initiated with multiple options
6. Side with Lucien during the three-way conversation
7. Return to the SMC and push Olivia into the reactor
8. Turn on the reactor and return to your ship
9. ADA patches you through to Lucien who instructs you where to deliver the journal
10. Go to Byzantium and place the journal in the dead drop, a mailbox
11. Turn around and talk to Simon Rentoe
12. The achievement pops upon conclusion of the conversation

Since the Postman path requires you to restore power to the Adrena-Time synthesizer, if you have not already done so, you will unlock the following achievement when you powered the reactor: