Bring your lease or ownership to the PO and they will change the lock within 30 days or you have to pay 35 dollars.
I'm not sure precisely, but if it helps, I got my mailbox key here in the first place. Perhaps call and ask if they can make another key or change out the lock. This probably doesn't help much, but I try!
Go to the Post Office that serves your address and ask for one. Take ID for proof of identity that shows your address.
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