Old-fashioned invitation enclosure, in brief

SASE

Think of what you’d send with a letter to ensure a reply comes back prepaid.

This clue refers to a "self-addressed stamped envelope," often abbreviated as SASE. Back when people mailed invitations or submissions, they’d include one so the recipient could easily reply without paying for postage. The "in brief" hint signals the answer is an abbreviation. It’s a throwback term you’d still see in old etiquette guides or writing contest rules.