Interesting idea - how can this be accomplished? Require the purchaser to send an iTunes receipt and the email address of the friend (to whom a promo code is se

Interesting idea - how can this be accomplished? Require the purchaser to send an iTunes receipt and the email address of the friend (to whom a promo code is sent)? I can't think of an automated technique, so perhaps that's the only way. Is there a smarter solution I am unaware of?