I need a way to share my app to allow people to download it for free with a coupon code or promo code or checkout code. I would like to post the code to a board, and invalidate it after some time. My app uses licensing and in-app billing, so mailing the APK may not be appropriate. The last question I saw regarding this was 6 months old, so I did not know if there was a newer solution available.
There is not a way to do this with the current feature set in the Play store.
You could produce a similar effect using the Licensing service (http://developer.android.com/guide/market/licensing) and perhaps in-app purchases. However sadly you will have to do most of the work yourself.