Note, you will be prompted to redeem or "enter" the promo code when you click on the link. Then pick the volume of your choice, proceed to checkout, and the volume will automatically discount to free.
Editor's Notes & Price Research
Offer is valid until 11/15/16. The limit is one offer per account. -qwikwit
And enter in code: AngryBirdsSonic
$4.99 will be deducted off the season price.
Code can be used once per account. Use Google family sharing to get all the seasons.
Note: After entering the code, pick one season to purchase. Click "$4.99 HD" button, it will show the deduction on the next screen. You have to click "Add to Library" on that screen to make the actual purchase.
Thanks to j0shed for the heads up.