Imo, 7pm is not late for a downtown NYC station. I would expect the customer window to be open even later there - especially "back then", although I have no hard data. Even 20 years ago, I could go to the airport mail facility in Indianapolis which had 24-hour window service and get service from a live window clerk at 3am if I desired. Many large cities had a 24-hour window somewhere.
That said, most mail is initially canceled during fairly normal hours and into the early evening. Finding cancels between 9pm and 6am are decidedly less common. Also remember that just because the customer window closes at a certain hour does not mean the action stops in the back room to cancel, sort, and move the mail c1931 by truck, rail, and plane all during the night. Just as much action happens after the window closes.
Here are some night-hour cancels, which could easily be brought in from carriers at the end of their route, deposited in street boxes collected one last time late each evening, etc.
And these early-hour examples, when backstamping was still practiced on first class mail, arriving at the destination in the middle of the night.
Yes, I known I still have no idea of the hours of Times Square Station in 1931.