Sealed Letter
noun
A seal in writing is writing that has been sealed into an envelope. This is a sealed letter.
ふうしょ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
契約書にサインをして封筒に入れてありますが、まだその封書を郵送していません。
I signed the contract and put it in an envelope, but I haven’t mailed the sealed letter to them yet.