Has the Agenda for the AGM been issued as yet, because if not, you could yourself add Motions requiring the most recent quotations to be provided in support of the E/C’s Motion, and to have the letterboxes attended to?
If the above is not possible, then you should immediately write to the Secretary of your E/C to formally request that it “exercises its function by causing the Owners Corporation’s (O/C) property within Lot *, specifically the ceiling in (location), to be put in a state of good and serviceable repair within 2 months of (letter’s date) and in accordance with the advice already received from its Contractors and as given in their reports”.
If that’s not done in accordance with the past advice and reports commissioned on behalf of the O/C, and you don’t receive a written explanation from the E/C as to why that’s the case, then you can apply for Orders under Sect 138 on the basis that your E/C’s inaction has caused the O/C to breach Cl.2(b) of that Section.
I know that this saga has been going for 2 years, but your E/C has been responding albeit in an effective way, so you have to put the “acid” on them in the terms suggested above so that Orders of the scope allowed under Sect 138 can be successfully sought.
Once resolved, only then try having the letterboxes attended to using that same approach.