The all-win situation is a great way of marketing your product. Make sure your product is good enough so you won't let the sender down. Also, make sure the gift is worth the sender's effort while you minimize the work of composing the email for the sender to send out.
Use "mailto:" HTML tag to pre-compose the email, set the b.c.c. to yourself so you also get a copy every time the email gets sent out. Then, you process one email a time to deliver the gift to the sender.
Here's an real case example I just received using this approach: