You are correct in that there is a threshold on the amount of property damage that automatically requires a police report.  Oftentimes, that amount is over $500.00 in total damages.  If your mailbox has been damaged, including the post, and the person's vehicle was damaged, I would ensure there was a police report filed in this matter.  If this means you need to speak with the captain or person in charge, then push the issue because ultimately your police report will help you in filing any insurance claims (whether those insurance claims are through your homeowner's insurance or the driver's motor vehicle insurance).