by Randy Carey:
There is nothing to "opt-in" to if you don't have an overdraft protection product.
by Brian Crow:
VISA/Mastercard networks do not permit chargebacks if the merchant obtained a valid authorization. If the account is overdrawn at the time a debit card posts, but there were sufficient funds when the authorization occurred, the transaction must be posted to the account. A chargeback may only be filed to return a debit card transaction if the merchant forced a charge through on an overdrawn account without obtaining a valid authorization.