If you lost your mail key, you can either contact your local post office for help or call a commercial locks company. If your office mailbox has been lost or stolen, you can contact the postal service for a replacement. The US Postal Service charges a standard fee of approximately $20 for replacement mailbox key tags. If your business owns its mailbox, then you can contact a commercial locksmith for help with a new mailbox key or new mailbox lock. If you’re lucky enough to have a spare key from your landlord, you can probably get one from them. However, if you need a commercial locksmith to do it, they can come directly to you.