Never again

I will no longer go to Volvo Unionville. On November 6th 2014, my wife bought our Volvo XC-90 to Volvo of Unionville to have them do a diagnostic check on our vehicle.
When Mark first called me, at approximately 4pm that day (6 hours after we left our vehicle with them) he notified me of approx 1 inch of water in our vehicle that caused our sensors to not work correctly.
I then asked if they had reset the sensors, to which he confirmed they had, and he informed me that none of the sensors were working (note: I only wanted the car to be brought in for one continuous error message showing up, which was related to anti-skid service.
The service manager then asked if he could remove the seat and look further and that it would only take another hour. I asked if I could pick up the car that same day, around 6pm. He told me that I wouldn't be able to pick up my car until the following day.
The next day came and my wife followed up late-morning and asked Mark what was happening regarding our vehicle. He then informed my wife that, to fix the sensors, it would cost around $1350, including the 2 hours for the diagnostic.
This is where my point of contention comes in; he never mentioned, at ANY point, the cost for the additional hour to remove the seat of my car for a "better look".
Which makes me wonder how they knew there was an inch of water causing the sensors to blink.
Also, because they took longer than necessary, I was charged 2 days for a rental, when we should have only needed a rental car for one day.
The rental company we obtained a car from is Enterprise Car Rentals, who are their associated car rental company.
i've paid $380 (for diagnostic costs and car rental). This is unreasonable as the diagnostic tool takes 15 mins to complete and shows where there are problems.
To remove a seat takes another 10-15 mins.
I do not understand and do not fathom where the 'two hours' diagnostic time, which they charged us for, came from.
Nor do I appreciate having them inform me that they need to remove one of our car seats, in order to establish "water damage', to which I agreed to, only to be surprised by double the cost when it came time to pick up my vehicle.
When I protested the surprise extra charges, I was informed, to paraphrase; "we will not reinstall your car seat AND you will not get your vehicle back until you pay us the full amount we are charging you now".
We have also spoken to Volvo's Ontario Head Office and they response was that they are 'unable to help us as the Volvo Of Unionville dealership is an independently-owned business. They claim they have zero authority in this matter.