What happens at a lockbox?

[menina 50]

Several weeks ago, I  sent in time sensitive paperwork for job re-certification. Following the instructions on the form, I sent everything to their PO box using USPS certified mail. I tracked the progress online and saw when the package arrived at the PO box. 2 hours later, the tracking changed to show that it had been forwarded to a different address. When I contacted the post office, they verified that the package had been sent to another state. I called the re-certification help line and was told that the post office forwards mail to a lockbox. The USPS tracking shows no record of the package arriving at such lockbox, and I have not received the return mailing receipt (green card) for certified mail. Nor has my check been cashed. The help line people are useless and keep telling me to wait 4-6 weeks by which time the deadline will have passed. My question is: what happens at a lockbox? Is it like a post office box where a note is left until someone can come pick up the package, or are people physically there to receive mail? Wouldn't the package need to be scanned again when it arrives there or when it is picked up since it was sent certified mail? I'm hoping someone here has some insight since a lot of the immigration paperwork goes to lockboxes and maybe had the same experience. USPS is also usesless saying the last known location was when it was forwarded. Thank you for any advice or help!