The phrase "afternoon post" is correct and usable in written English.
It can be used to refer to mail or deliveries that occur in the afternoon.
Example: "I will check the afternoon post to see if my package has arrived."
Alternatives: "afternoon mail" or "post delivered in the afternoon".
The Times, a morning paper, toyed with the idea of creating an afternoon daily to go head-to-head with the afternoon Post.
In London on Monday afternoon Post plans to eat the hamburger in front of reporters and anyone else in the world who wants to take a look, via a livestream of the event on the internet.
"The very foul-mouthed Sen. John McCain begged for my support during his primary (I gave, he won), then dropped me over locker room remarks!" Mr. Trump wrote in an afternoon post on Twitter.
According to the narrative of the poem, Plath wrote Hughes some kind of suicide note, or a note hinting at the possibility of suicide, on the Friday, and by some perverse miracle of the Royal Mail it arrived too early: she posted it in the morning and he received it in the afternoon post.
it's a good Saturday afternoon post.
Since she's been able to talk, every afternoon post lunch she'd ask excitedly, "nap?
My afternoon, post-massage, was largely spent wandering around the most ostensibly harmless fields I could find, attempting to recover my lost innocence.
He had coached his first game and lost his first game, and now he was facing his first Monday afternoon post-mortem.
NetSuite this afternoon posted better-than-expected results for the first quarter.
Dutta this afternoon posted he may be releasing a Chrome extension as well.
Lovato's aunt on Tuesday afternoon posted on social media that the singer and actress was awake and responsive.
