Phil, I hate to say it, but they've got you over a barrel. If you don't pay, they can keep the cab.
Unless you have a contract that specifically said that they had to notify you of any additional charges, you're stuck with the bill. I think I'd definitely let them know you're not happy getting stuck for almost twice the original quote without any notifications, and you won't be recommending them to anyone else in the future. I would also ask them to discount the price, since they gave you no warning of the extreme overage.
When contracting for work like this, you should have a clear understanding of what the contractor will and won't do, both in regard to the work and in regard to keeping you updated on the work and pricing. If the job goes one dollar over the quoted price, you should be notified
before any further work is done, and you should have to OK, in writing, any additional charges. This protects both you and the shop from misunderstandings.