You are not stuck with them unless you agree on it.
Whole US has the same laws regarding contracts and states can add only small print to it.
Negotiating is never easy task, but if the shop gave you firm estimate on the job and finds later it takes longer, it is their problem to eat.
On complicated jobs, smart shop owner will not make estimate before inspection and they can charge you inspection cost, but that is all they can before you agree on more.
Not sure if that applies to automotive contracts, but construction law in CA doesn't allow down-payment bigger than 10%.
Anybody taking more can loose his licence very fast.