Anything you submit is going to be based on mitigation, so you need to submit something non-confrontational and persuasive.
The main point is clearly the fact that you are an Autopay account holder of some length of time, (7 years), and have never defaulted in all that time, but you missed updating your account when your debit/credit card expired. Emphasis that there was no intention to avoid payment, and ask for the PCN to be cancelled on a goodwill basis. Also state that you will obviously pay the unpaid toll when requested.