Payed $0, and got the code 20 seconds later.
Good question. If you look at a page that has the promotion (under Special Offers and Product Promotions at the bottom) and click on "You will receive your code by email within 2 business days. Here's how (restrictions apply)", the last line says Offer only applies when shipped and sold by Amazon.com.
Also, if you scroll to the bottom of that page, you will see that the promotion is not listed under special offers and promotions.