I wonder if that's a current sign or if it's been sitting there over a block in the wall that formerly housed a vending machine years ago. According to the internet, stamp vending machines were removed from service between 2006 and 2010 and Automated Postal Centers (a/k/a Postal Kiosks) replaced them in high traffic areas.
The rationale for removing the old vending machines is recited in this 2006 excerpt from the Postal Bulletin:
https://about.usps.com/postal-bulle...txt.html#one