This is a USPS question.
When you close escrow on your purchase, you'll take your closing paperwork (final HUD-1) to the Fort Mohave Annex in the Safeway Center. They will then give you a cluster box key and location finder.
A USPS POSTAL BOX IS FREE, BUT YOU MUST PROVE YOU LIVE THERE OR JUST PURCHASED THE HOME TO USPS, IT TAKES ABOUT A WEEK FOR THEM TO SETUP YOUR BOX AND THEN GIVE YOU THE KEYS. THE POST OFFICE IS BEHIND SAFEWAY AT AZTEC AND HWY 95.