Let's restate what GoGreen has asked.
Our bank does not have a formal program but implements an approval balance at the POS and ATM that includes a "Courtesy Pay" discretionary approval. Can we still do this option if we get an opt-in form?
If you have an approval balance used for authorization of ATM and POS transactions and that balance includes a "courtesy pay" padding amount, you will occasionally be approving an ATM or POS transaction that overdraws an account. That's OK under the OD Fee rule in Regulation E if you don't assess an overdraft fee for those transactions if the account hasn't been "opted in."
Of course for non-opted-in accounts, you also will not be able to assess an OD fee for an ATM or POS transaction that sneaks in for whatever reason and overdraws the account.
If you send the opt-in disclosure (A-9-like form), receive an opt-in, and provide a confirmation, you will be able to impose an OD fee for ATM and one-time debit card transactions.