There are five post offices in or near downtown Springfield. At one point there are three along Main Street within one half-mile: the MPO, just off the highway; Central [Tower Square] Station; and Colonial Station, which is on the consolidation 'Hit List', meaning that it'll likely be closed in the near future.
Central [Tower Square] Station is interesting in that while it's at the heart of a relatively large city, it's open for just an hour and a half each weekday. It's essentially there to provide service to a prominent downtown office building. The station features PO Boxes. The clerk, when done at Tower Square, then moves to another office in Springfield -- whichever needs the most assistance that day.
Frankly, I think there should be more such stations. If the clerk moves from place to place, the marginal cost of providing the additional service is minimal!
I decided not to try to hunt [read: pay] for parking in downtown Springfield, so I found a spot at the Springfield MPO and walked to Tower Square first; then the Colonial Station; and then back to get the postmark from the main office windows. The parking sign said "for postal customers only", and I definitely fit the bill even if I wasn't always at the main office!
Without further ado, Springfield, MA's Central Station:
The main post office is long and bland.