The PC version wasn't available previously, only the PS4 and X1 versions.
UK Links:
PC - https://www.amazon.co.uk/505-Games-ROCKETPC02-Rocket-League/dp/B01G8H8AVO/
PS4 - https://www.amazon.co.uk/505-Games-ROCKETPS402-Rocket-League/dp/B01DVP2MQU/
Xbox One - https://www.amazon.co.uk/505-Games-ROCKETXBX102-Rocket-League/dp/B01DVP2N32/
USA Links:
PC - https://www.amazon.com/Rocket-League-Collectors-PC/dp/B01FVTE1U4/
PS4 - https://www.amazon.com/Rocket-League-Collectors-PlayStation-4/dp/B01D9T0A1C/
Xbox One - https://www.amazon.com/Rocket-League-Collectors-Xbox-One/dp/B01D9T0BAC/