Walmart offers preorders of Marvel's Captain America: Civil War on Blu-ray Disc for $19.96 with free shipping. (Amazon and Best Buy offer it for the same price via Prime and in-store pickup, respectively.) That's the lowest total price we could find by $5. It features the theatrical release of the movie, plus commentary, gag reel, and extra and deleted scenes.
Of note, Target offers preorders of Marvel's Captain America: Civil War Collector's Edition on 3D Blu-ray / Blu-ray / Digital HD bundled with a $5 Target Gift Card for $22.99 plus $4 shipping. Assuming you'll use the gift card, that's $6 under the lowest price we could find elsewhere.
Both titles should be released on or around September 16.