Amazon has the 28 Days Later/28 Weeks Later double feature for $7.88:
https://smile.amazon.co
Shipping is free on orders $25+ or for Prime members. Choose no-rush shipping for a $1 digital credit (or, potentially, some other credit).
Alternatively, Walmart also has this for $7.88:
https://www.walmart.com/ip/28-Day…y/31963853
Shipping is free on orders $35+ or choose free in-store pickup (where available).