envelope

/'envələƱp/　noun

★ a flat paper container used for sending letters in, with a part that you stick down to close it

・Writing paper and envelopes are provided in your room.

・a padded/prepaid envelope

container n)入れ物･容器･コンテナ

pad　n)綿などを中に入れた詰め物　　padded envelopes  クッション性のある封筒

prepaid　adj)料金前払いの　prepaid envelopes 料金先払いの封筒

Did you put coupons in all of the envelopes before they 封筒 were sent out?