depends on the carrier.
but let me say that as a former person who had to call carriers all the time, they're required to be at work at like 8:30am (if you had a complaint with your post office, you'd be able to talk to your carrier on the phone at that time. they leave at like 9:15).
i'm not sure why they'd be delivering mail at 6-7pm... it doesn't sound reasonable. but since i'm not in their shoes and don't understand national routes, i am not going to comment further on that matter.