EDIT: Price has gone up to $14.99 now :( still 50% off, just less of a bargain now, you can still get the pack with original and expansion for $19.99 as well
Best price this expansion has been yet
Amazon do have a pack with the original and this expansion for $19.99, but while the expansion is on sale you would be better to pick up the XCOM Collection which has all the classic XCOM games, and XCOM: Enemy Unknown and some DLC for EU for $9.99 here (this collection is one Steam key)
As always you will need a valid US address on your account to purchase digital games from Amazon, personally I choose to stay at the LVH when I buy games. It has been noted that you should pick an address in a state where Amazon do not charge Sales Tax list here
Game of the decade. I am quite literally addicted. Can't recommend highly enough!