Check out
Moviesanywhere.com if you're US based. It originally started as Disney Movies Anywhere but later expanded to additional movie studios. Basically, they authenticate your purchases from specific studios (Sony, Universal, Disney and WB) from any of their partner merchants (Apple, Google Play, Amazon Prime, Microsoft, VUDU, xfinity, Verizon, DirecTV) and add it to your library on ALL of those sites. So, if you buy Star Wars from iTunes, it will be add to list of purchases/library in Google Play, Amazon, etc). It's not a subscription service--completely free, and they linked purchases retroactive to you actually signing up with Movies Anywhere.