Car rental companies contract with private companies to collect tolls and tickets for traffic and parking violations. There is commonly outrageous pricing related to this and the companies often have official sounding names. For example, Avis/Budget uses Highway Toll Administration which has a official looking logo and official sounding name but is just a regular for-profit company. CVM is just another of these companies.
It’s possible whatever pre-payment you made for the rental vehicle didn’t properly associate with your rental, or for whatever reason wasn’t exactly correct, and now you’re being billed & possibly fined for non-payment. It’s also possible the vehicle had a toll transponder which was billed and your payment didn’t apply because it was based on the license plate. If that’s the case, you’ll need to see if there are specific terms that cover this situation.
The bill from CVM should include information about specifically what you’re being charged for so you should be able to confirm it matches whatever you actually drove. Assuming it matches the time you had the rental car, the only way I can see getting out of the payment is if you can prove that you properly prepaid and successfully dispute the bill with the toll road operator (The Toll Roads of OC).