This is a UK only deal.
If you’re in the UK, you can use coupon code KARKA-NDDLC-CODE1 to get 15% off the Back to Karkand DLC for Battlefield 3 at Green Man Gaming (GMG).
Would make it around £10.17 after the promo code. Not necessarily the best deal but currently this is the only deal I know that’s not from a CD key type site.
Should activate on Origin.
Back to Karkand has a release date of December 13th for the PC.
Edit: BTW, if you pre-ordered BF3, you automatically got the LE edition, which means you get BTK for free.