The US Postal Service is operating on extreme debt and can't do anything about it except fire more employees and close more offices. They can deliver mail anytime between 7am and 10pm. The standard is 5pm but they have permission in about a quarter of the country to deliver after 5pm and as late as 10pm.

We usually get our mail between 6pm and 9pm for the last three months because the three closest post offices were closed in December.