in the post

idiomatic

(idiomatic) On the way; soon to arrive.

Japanese Meaning

（郵便物が）発送済みで、もうすぐ届く / 配達途中であることを表す

Don't worry — the invitation is already in the post, so it should reach you by Monday.

心配しないで — 招待状はすでに発送中なので、月曜日までには届くはずです。

Sense（1）

(idiomatic) On the way; soon to arrive.

We have survived our trials; yours are in the post.