You should see Redeem a gift card or promotion code or something similar. like View balance.. your account should show $20.00 credit for the digital goods. This is right under the pricing for season 3 from my example
The code only works for buying gems in game sorry cant stack with amazon coin+gem discounts
If you messed up and bought amazon coins dont freak out amazon live chat can refund your money back to you =]
As a previous winner and have gone through it with Amazon customer reps. =]
You cannot use the amazon giveaway codes to buy Coins. Only in app purchases
http://www.amazon.com/gp/product/B00JJ2T94I/ref=dv_web_a_yt_s_l_2_7222?pf_rd_p=1976662362&pf_rd_s=center-6&pf_rd_t=101&pf_rd_i=2858778011&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1NX8A53ADA4SMGQ4T965
The code only works for buying gems in game sorry cant stack with amazon coin+gem discounts
If you messed up and bought amazon coins dont freak out amazon live chat can refund your money back to you =]