The Emperor was the 2nd mail-away figure available on Return of the Jedi cards. It was also really the only major character from the movies that was offered as a mail-away. It is also the easiest to find in an unmarked mailer box. I picked up one of these originally in the early 90s for around $6 for a minty figure in my collection. They now sell for roughly $10 on ebay, so still fairly cheap. For my mailer collection though I wanted to track one down with a mailing label, so it took a little more patience and I actually got it for the $6 price, so I was pretty happy about that. The Emperor required 5 proofs of purchase and the offer expired on April 15th, 1984. Here is how the offer appeared on card backs.
Here is my emperor with mailer box and catalog
and finally the T.V. commercial advertising the promotion