USPS don't do Sundays. Unless you are very friendly with your route delivery person and suspect a secret visit from him/her off hours???
It was probably dropped off to your neighbor and they brought it over, unless you overlooked it yesterday.
Before I moved, my tractor mounted snow blower had a worm gear go out. Saturday, on his way home, my USPS man dropped the box of parts off in my mail box as he knew by a unblown drive way I needed them.
I seen him drop them off as I was out shoveling. Otherwise, I would have never known until Monday he dropped them off Sat. on his way home.
My dad worked at the post office for 35 years. I know all about what the good guys do, and the not so good guys.
My dad was a good guy. He did a lot of after hour drop offs out of his way on such things when his customers were in need of a parts package.
That's back when local farmers were still part of the local econemy.
Did I ever tell you about the farm I worked for making hay that let his daughter go shirtless?
True story, for another time.