To receive the Columbia House Selection of the Month, do nothing.
"Free" music, circa 1978
The form had a "return by" date and several boxes to check, one that read "Do not send Selection of the Month". Miss the deadline, or fail to check that box, and a tape (or album) not of your choosing would be mailed to you automatically. Your account would be billed $9.98 (or more). And the "Would you believe..." sales pitch of "13 records or tapes for $1.00" would begin to make sense. You were a fish, on the hook for 9 more selections at regular club prices.