If you login rocket league from steam,plz leave your steam link in order details.
If you login rocket league from epic,plz leave your epic id in order details (epic id: your epic id)
If you login rocket league from epic but ur inventroy belongs to PS4 or xbox rocket leage,dont order from this offer,this offer only for pc rocket league.
If you are new player, free to play Rocket leauge,Before you buy credtis at this website, make sure you have purchased at least 500 Credits from offical store,so you can trade with others.
plz check this link for more details