I suppose it's best to start with what once WAS the Bay Shore Main Post Office, now known as Bay Shore's Penataquit Station. Built by the Treasury Department during the '30s, it's a WPA post office, complete with a piece of commissioned relief sculpture and a beautiful wooden counter area with a great sign above that says "Bay Shore". (The font is quite unique and lovely.)
Believe it or not there's a Wikipedia page devoted to the Bay Shore Post Office, though it's a mistake to still call it that. As the page will attest, the post office was one of many WPA facilities to be added to the National Register of Historic Places, in 1988.
As the town expanded, the post office needed more room to operate, and primary operations were moved to the outskirts of the town in the '60s (or, what appears to be the '60s based on the architecture of the building, shown below). It's quite a nice building: white brick with light blue accents.
Thus, the "original" [historic] Bay Shore post office is now its downtown Penataquit Station.
The best part of the new office, in the author's opinion, is this wonderful mosaic featuring the Post Office Department logo:
Well, almost. It WAS a beautiful mosaic with the old logo; however it now features a bolted Sonic Eagle [new USPS logo] on top of it. Some people have no sense of posterity: