The headline is misleading. The case involved a landlord who wanted the post office to deliver to a shared postbox that she made herself for her tenants. The post office refused because it wasn't suitable, i.e. it was shit. The Supreme Court simply ruled that she couldn't force the post office to deliver to just anywhere.
The facts of the case are as you say but the case was decided by a much broader issue, sovereign immunity. In other words, no one can sue the post office for anything, even intentional acts like refusing to deliver the mail. The effect of the decision is much broader than a beef over a custom fab mailbox.
Held: The United States retains sovereign immunity for claims arising out of the intentional nondelivery of mail because both “miscarriage” and “loss” of mail under the FTCA’s postal exception can occur as a result of the Postal Service’s intentional failure to deliver the mail.
Postal Service v. Konan (24-351): https://www.supremecourt.gov/opinions/25pdf/24-351_7648.pdf