You have to be a resident of Maine to buy a gun there. Having a post office box doesn’t prove that. There aren’t any guarantees that the record is destroyed. I’m sure they take at least daily backups of the database. They put that on a tape and send it somewhere. Usually you would keep at least 6 months worth of backups. So, even if some process runs to remove your record from the current database after 72 hours your record may exist on a backup.
Thank you