Your wife can sign in to Easy Pass website to upgrade to Easy Pass Plus to start using M-Flow lanes.
Easy/M-Pass are prepaid systems with transponders. They use the same lanes so the fine must be from her using M-Flow lane, where the cameras read the car number plates and auto-debit the toll from the bank if she had registered on M-Flow system. If not, she needed to sign in to their website and pay on time, late payment will carry a hefty fine. With Easy Pass Plus, the system will auto pay just like she had registered on M-Flow.