On Normal working days the USPS delivers until close of business which is usually 6:00 PM but can sometimes be 5:00 PM depending on where you're located.
So on those normal business days the USPS will stop delivering packages as of 6:00 PM at the latest.
But I've noticed when there's lots of Holiday packages to deliver such as for Christmas then the USPS will work overtime and deliver as late as 9:00 PM in some places.