It sure is ok. It's also ok for a customer to disagree and leave. First up, ensure they'll stay once you start billing for this in the future. Next, see if th

It sure is ok. It's also ok for a customer to disagree and leave. First up, ensure they'll stay once you start billing for this in the future. Next, see if they are willing to cover the cost of services provided in the past that was unbilled due to your error. It's good to offer some incentive (like your back-charge for 6 months only) to make it clear that your mistake caused this to go unnoticed.