It may seem odd for the USPS to be in the business of buying ammunition, but actually it has had armed officers for centuries. Chances are, this buy is for the US Postal Inspection Service. The USPIS employs about 1,600 special agents who investigate mail fraud and related crimes. Some of these investigating agents pack firearms, usually a Beretta semi-auto 9 mm pistol.
I live a few blocks from a huge USPS facility. There are always a few USPP cruisers rolling around on the neighborhood streets.