Envelope sent by FinnBadger (the USA), with a special pictorial postmark that celebrates Pi Day. And, of course, this had to be accompanied by a round stamp!
Did you know that Pi Day was a thing? And it also exists the Pi Approximation Day, which happens to be today, since the fraction 22⁄7 is a common approximation of π.
Have you ever sent/received π-related mail? I have just read on the Wikipedia:
Massachusetts Institute of Technology has often mailed its application decision letters to prospective students for delivery on Pi Day.