In my area the USPS mail is delivered to addresses by 3:00 PM but starts as early as 8:00 AM
Sometimes during holiday rush deliveries the USPS will deliver as late as 9:00 PM in my area but most times they stop delivering by 3:00 PM in my area.
USPS in other larger cities and locations may continue to deliver packages and mail until 6:00 PM but it depends on the location.
If you don't get your USPS mail or package by 6:00 PM then you'll likely get it the next day.
USPS will deliver all the packages they have on their delivery trucks as soon as they can.