Found it Ocean Players Club – Global Casino Rewards Program
It does say this in the correspondence:
Please note: At the end of every voyage, we process any funds that were left in the player's bank account. Funds in excess of $25 will automatically be issued via a check to the address we have on file in a Carnival Cruise Line envelope.
Any balance below $25 will be processed manually upon request
If you still have not received the outstanding balance check, and it is outside of the 6 weeks, please submit a new request: Balance Inquiry. Our agents will review your case and advise any updates regarding your check.
Please provide the correct mailing address within the form to verify it was sent to the correct address.