https://www.amazon.co.uk/Square-Enix...ds=heavensward

https://www.amazon.com/FINAL-FANTASY...ds=heavensward

both amazon US and UK sell PS4 heavensward so I assume others will too

You are not forced to buy the expansion at 50, however you will not be able to access any of the expansion content and zones, you will be stuck with any content that came out in ARR plus any additions permitted to ARR players like palace of the dead