What's the (current) process for getting Arrow locks fitted by USPS?

I’m not sure this question is at all appropriate for DIY.stackexchange, but it seemed the best fit so if it’s not appropriate here it’s probably not appropriate anywhere. Perhaps this is a question I should just ask the USPS directly, but I wanted to first try to collect some third-party experience with the process.

Brief background

We have a 9-unit apartment building with USPS-approved mailboxes mounted in the secured entry lobby. To deliver mail, a copy of the building front door key has been provided to our regular mail carrier. Problem is, that doesn’t help at all if a different USPS employee needs to deliver mail. (Say, dropping off a Priority or Express package, or even just someone else covering our regular guy’s route when he’s out sick or on vacation.) Without that One Key we’ve provided to the USPS (really, one specific individual employee), the mail can’t get in.

My (possible) plan

I’ve already ruled out "more keys for the USPS!" as an option for solving this — that way lies madness. Fortunately, there’s already an existing (and much better) solution, which I’ve seen employed at other residential locations: An exterior box, sometimes called a "key keeper", mounted outside the front door and containing the door key on a chain. When fitted with a standard USPS "Arrow lock", any postal carrier can open the box, then use the key stored inside to unlock the door and enter the building.

You can buy the actual boxes from many sources (here’s one version from usmailsupply.com), but as one might expect they come without the lock installed.

What’s next?

So my question becomes, before I consider ordering one of these: How do you go about getting the actual lock installed? Is it a service provided by the USPS directly? How does one even go about making an install request? Are there fees involved?

While it’s easy to find the hardware for sale (sans lock), my not-inconsiderable web skills have failed to find any information whatsoever about the process of getting the box outfitted with a lock. I’d have thought there would be a form and/or documentation somewhere on the USPS site, but all I’ve come up with is an almost-suspicious lack of any information whatsoever.

I know that USPS policy has been shifting towards centralized delivery in recent years, so that they no longer have to visit individual addresses in order to deliver mail. And perhaps part of that shift is phasing out installs of Arrow locks on individual building entries and mailboxes. But we don’t have centralized delivery here, and if the carrier is willing to carry a copy of our building key… ¯_(ツ)_/¯