Strictly speaking it’s an improvement and so it needs a SR.
In the REAL world just pass a motion approving the cost at a EC or GM and it shouldn’t be a problem. The mailboxes are the Strata responsibility.
I don’t see how you can force people to lock their mailboxes unless the flap locks automatically when the flap is dropped. I don’t lock my mailbox but I have a PO box and nothing important goes to my home add.