An abbreviation of '口座振替' (account transfer/direct debit). Often seen on bills to indicate automatic account withdrawal.
電気代の明細に『口振』と印字されていた。
The electricity bill had '口振' printed on the statement.
コンビニでは支払えません。口振でお願いしますと案内された。
I was told I couldn't pay at the convenience store and to use direct debit instead.