We give our packages when ready to the mail truck at noon or so.
The rest go to the PO at 5:14pm they close at 5pm and last call is 5:15pm.
We put them in the bin, unless we have a lot, they are in a tub, and the mail people appreciate us not filling up the bin.
Our PO has a buzzer on the door near the bins. We push that get a smile, or a little ribbing that “we are late once again” they take the tub and scan everything in for us.
#1 advise for a new seller, if you are using USPS make friends with the Postmaster and workers. Our postmaster even found a package on Christmas Eve that we put in the phone number in the address line, in a dead letter bin by another postmaster, who drove it to the AZ customers home.
Get to know and appreciate your Post Office People, or find a small post office nearby and do the same.