People tend to check in during these times:
Line always way too long.
If you are just buying stamps, mailing letters, or shipping small packages, use the APC machine in the lobby. It takes credit/debit cards and is open 24/7. Never a long line
If you come anytime after 10 be prepared to wait in a long line. While one clerk tries to take care of everyone. I wouldn't be in the best mood either...
Watch out for rude clerk had to report her!!!