Thumb Score: +13

Amazon [amazon.com] has 6-Pack 16.9-Oz Diet Coke Diet Soda 4 for $21.56 – $15 (multibuy discount) – 5% when you check out via Subscribe & Save = $13.92.
Shipping is free with Prime or on $35+ orders.

Savings
Get any 4 for $15 (check and activate [amazon.com])