postcard with a reply card attached

(formal) friend

postcard

to use / to employ / to make use of, to apply / to spend

a round trip, going back and forth / (by extension) an exchange (of conversations, letters, etc.) / coming and going

a round trip, going back and forth / coming and going / (by extension) exchange (conversations, letters, etc.)

reply, response

to reply, to respond

to send; to dispatch / to see off, to take (somebody somewhere), to escort / to play back / (linguistics) to add okurigana / to pass the time / to see off a deceased person

To send