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

1. Apply the gift card code to your account. Your balance should show up as 0.00 but redeemed
2. Go to any instant movie example 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
3. 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 =]