Okay, let me help everyone out here.
Okay, first you E-mail Slackbot at the E-mail address she posted above. When you receive her address through E-mail, you can start on your postcard. You will have to supply YOUR OWN postcard, and on the address line of the postcard, you will put the name of the performer. Then work on the letter. Put in words of appreciation, thanks, etc. You will then put the postcard in an envelope addressed to the address Slackbot gave you, so she can pass it on to the performers without having her address on the postcard. Put proper postage on it, and mail it. Slackbot will give the performers the postcards, and what happens next I don't know.