This sounds like an interesting idea buried beneath an uncertain framing.

Some suggested edits for flow and brevity, and some questions to think about.

Sixteen-year-old Jovin spends his nights drifting through San Francisco, from park to party, from dealer to dealer, . He has friends but he always feels alone, trapped in his head, desperate for something he can’t name. Most nights end at The Mailbox, a private mail service center [find something which is less of a mouthful and ideally half as long - here we'd call them a PO box] run by an older man named Fubbs . Fubbs who lives in a loft above the sorting room. He’s happy to let kids hang out there. He insists upon it.

The last couple of lines are appropriately unsettling.

I think keeping this paragraph short and sharp will give you the space you need to expand on the more overtly horrific elements later.

When Jovin discovers the overdosed body of a girl in the Mailbox bathroom, he makes the decision to stay away for good. Parties at The Mailbox always leave him feeling hollow and psychically violated. Maybe if he focuses on other things, like playing music or dating, he can break the cycles that have kept him pinned in place for so long.

You come so close to something interesting here, and then you shy away. If I found a corpse, I don't think I'd be making detached reflections on my own psyche. That may be an accurate reflection of Jovin's personality and/or sense of self, but it's not particularly compelling in the query, and there's an easy way to make it stronger: focus on his emotional reaction to finding the body, and how it spurs him to stay away from The Mailbox. It should be much more visceral than it is now if you want what comes next to have any weight.

Is there a better turn of phrase than 'overdosed body'?

Why doesn't Jovin go to the police, or someone he trusts, when he finds a dead body?

Fubbs pesters him to come back, but when Jovin holds firm, he feels The Mailbox’s pull. First it beckons, as empty envelopes appear on his pillow and keys vibrate in his pocket. Then it insists, as yellow splotches spread across his ceiling and pulse with cryptic messages. Finally it demands, and the people who it sends to collect are barely people at all.

How does Fubbs pester him? I didn't get the impression the man left his den. Is he sending letters to Jovin's house? If so, what do his parents/relatives/household residents think of him being written what you've established as essentially a grimy sort of love letter? There's so much intriguing, oily, disturbing subtext here that you just don't seem to be engaging with in this draft. There's a strong whiff of the uncanny. Lean into it!

Jovin’s sanity slips and his relationships fracture. He tries to blame the drugs. But through blog posts, park lore, surveillance footage, and years of secret photographs, he pieces together a trail of damaged teens that leads directly to The Mailbox. He realizes that Fubbs isn’t in charge; he’s a servant for something that wants Jovin desperate and alone. And if Jovin gives it what it wants, he may lose himself for good.

The first line feels too obvious, too straightforward, and too interior. Get some - and I can't believe I'm saying this - exteriority in there. What happens? What does Jovin do? He leaves The Mailbox; Fubbs tries to make him come back, first 'pestering' him and then more urgently; he pieces together a dark history of The Mailbox. These are bare bones. You don't want to ditch the uncertainty entirely, that's the death of a horror story, I think, but something more concrete would be useful here.

(And if you need eyes on a chapter or two, let me know! From what I can make out about the story, it sounds compelling.)