Right before the holidays, Blizzard has released a new way for you to purchase StarCraft II. You can now in one package purchase Wings of Liberty and Heart of the Swarm in the new StarCraft II Battlechest.
Right now, the Battlechest retails for $39.99 in retailers just as Amazon. However, you can actually buy the 2 separately for cheaper right now. (Wings of Liberty is $17.93 and Heart of the Swarm is $20.46).
However, Blizzard is offering up the package on their digital store on sale at $34.99 so that is currently the best way to go unless you want the physical box. With Legacy of the Void formally announced at BlizzCon with a beta coming up in 2015, this is a great way to catch up on the story in anticipation to the protoss story line.