From the lobby, click 'Rewards'. There you can find 'Random weekly crate'.
Click 'RECEIVE' then you will be able to make purchase with BP.
Either a Wanderer crate or a Survivor crate will be randomly delivered.
The more crates you buy, more expensive it will get! The prices will be reset on 9am every Mondays.
1st purchase: 700 BP
2nd purchase: 1,400 BP
3rd purchase: 2,800 BP
4th purchase: 4,200 BP
5th purchase: 5,400 BP
6th purchase: 7,000 BP