Walmart offers preorders of Deadpool on DVD for $14.96 with free shipping. (Amazon charges the same.) That's the lowest total price we could find by $4.
Walmart also has preorders of Deadpool on Blu-ray / DVD for $19.96 with free shipping. (Amazon charges the same.) That's tied with our mention from a week ago and another $4 price low. Both include a digital copy of the film and are scheduled to ship on May 10.