If they wanted to restrict access to the post office, they'd have to screen for ID at the entrances.   You don't make the mailroom more secure by telling a student who has received mail addressed to her at the university and has been notified of this fact by a claim slip inserted in her own university mailbox, and who can provide both the valid claim slip and legal proof of her identity, that she can't have her package.   That she had a mailbox for them to put the claim slip in is in itself proof that she's a student at the university.