Billpay-Overdraft program or Extension of Credit?

We are about to implement a risk based billpay model in online banking. Our billpay vendor will make the payments on the customers' behalf, and then send us a file that we will post to the customers' accounts. If there are insufficient funds to cover any of the billpays, we will need to attempt to collect the amount from the customer due to the fact the funds have already been sent. Would this be considered an overdraft program or an extension of credit? Would there be any Reg O concerns associated with this process considering a Director could initiate a billpay that overdaws their account and if it is considered an extension of credit it may cause a Reg O issue. I do not believe this is governed by overdraft regulations and would not be considered an extension of credit.