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“The mailbox is rather old and is due for replacement”
It definitely sounds like you’re saying the cluster mailbox has reached the end of its life cycle in which case the HOA should be responsible for replacing it. This should all be found within your HOA’s reserve study and money should have been set aside in your reserve funds to pay for the cost of replacement. The board is responsible for this task and they are not doing their job right if you don’t have reserves.