Probably wouldn't cost you much more than that to rent a car and drive yourself. :shrug:
As far as your question goes, you never know. Sure, MAYBE, the person who originally helped you was new, and MAYBE she misquoted you, and MAYBE she'd get in trouble if you insisted on getting the price you were quoted.
But MAYBE this is not an uncommon thing on the part of Happy Limo and Mousesavers doesn't know about it. Not saying it happens every time, but every so often a quoted price goes up because the person working the phone that day was "new." And they figure people like you won't complain because you don't want to get an inexperienced worker in trouble for an innocent mistake and that's why they can get away with charging you nearly 50% more.
For me, the big warning sign is they call themselves "happy limo." Really? Is that what I'm getting? A happy limo? It's a cutesy-poo name meant to disarm you so you're not thinking about the fact that a happy limo might be taking advantage of you. 'Cause they're so HAPPY!
Checking out the website, I find it interesting that, unlike a rental car website where you can input what you want and they'll spit back a price, you have to request an estimate. When it comes to a company like this, I'd have to think they'd advertise a special rate for people going to WDW, considering that it's probably one of the main sources of their income. Even it winds up being a bait-and-switch deal (like, the price is for a town car, 4 people maximum so if you want a different vehicle with more passengers, it goes up), there seems to be no baseline, which means they can charge you whatever they want, and the only way you'd know if you were getting hosed is if someone else in your party made a similar request and you see how different prices might be.
Personally, I'd just call and cancel. When they ask why, explain you were quoted a price, and not only was that quoted price not honored, but you had to make the phone calls to find out why, without any apologies from the company that made the mistake in the first place. Don't say "now, if you offer me the original price, I'll keep my reservation;" any reputable company will do their best to honor a quoted price. If they say "fine, you're canceled," then you'll at least know that your quoted price was, indeed, too low. That means the price was SO good, they're willing to risk losing your business now and in the future, to say nothing of your bad word of mouth, because there's no profit in it for them. If they say "OK, we'll honor your quoted price," then either a: they were trying to hose you or b: they're willing to take the hit to maintain good customer relations.
or, as I only-half-jokingly stated above, you can see how much it would cost to rent a car. it might not be that much more