Amazon offers Prime members the Big Hero 6 on Blu-ray Disc / DVD / Digital HD for $18 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's a buck below last month's mention and the lowest total price we could find by $6. It includes a digital copy of the movie.
Don't mind picking it up? Target charges the same via in-store pickup.