Since USPS is closed on Sunday they won't be able to sort the package and send it on it's way nor will the USPS be able to accept the package directly from you.
So if the package will fit in the drop box at the USPS then you could drop the package off in the USPS drop box as long as it will fit and the postage has been paid for.
I have dropped some packages in the USPS boxes outside or inside the big drop boxes made for packages and they've always accepted it.
Although the package won't be scanned into the system until the next day when Monday comes.
USPS sorting facilities work on Sunday but the local post offices do not.