You probably have a World of Warcraft subscription. I remember reading an article about it a while back. I believe a 6 month subscription came with a bunch of HS goodies, but can't find the article find now.
For a limited time*, you can get a slew of Hearthstone goodies when you purchase a World of Warcraft 6-Month Subscription from the Blizzard Shop.
What you Get:
Once you’ve purchased your subscription, you’ll receive 15 card packs to United in Stormwind, a Tavern Pass, and Battleground Perks. You’ll receive an additional 15 Mercenaries card packs with the release of Mercenaries on October 12.
As an additional bonus, you’ll also receive 15 card packs from a future Hearthstone expansion along with 15 Mercenaries card packs, a Tavern Pass, and Battleground perks at a future date.