Letterboxes are Common Property, and unless your Plan has a Special By-Law to the contrary, the Owners Corporation (O/C) is responsible for all maintenance and repairs.
Depending upon the numbers of Lots / Letterboxes, it may be necessary for an Item to be placed on the Agenda for your next AGM so that your O/C can, on the basis of improved security, resolve to allocate sufficient funds for the necessary upgrade works to the Letterboxes and by the sound of it for improvements to the Building's security access system.
If the timing of your next AGM is too far away, it may be worth your while to upgrade the lock on your own Letterbox, and to seek a reimbursement from your O/C; just advise your Executive Committee Secretary or Strata Manager beforehand.