Are you on My Coke Rewards Beta? If you're not, you should definitely migrate on over. The rewards are somewhat more plentiful and different (such as Amazon being a redeemable prize) if you're at least silver. So get social, will ya?
For a very limited time (thru 9/19), MCR Beta members can enter 12 pack codes from Coke, Diet Coke, or Coke Zero. That means 20 points instead of 10. But this is only if you enter them on the Beta site. If you're not a Beta member, you should know you can enter codes on the regular site and the Beta site, but it all goes into your account the same way. However, the Beta site does offer some benefits that the other site does not (and this is one of them). So definitely come on over. I have an online friend who won a contest from the Beta site --- $200 prize, just recently.