Estimates are wide open. I expect some warning of extreme overages, but basically your only recourse is refusal of future business.
Quotations are understood to be fixed price. ... but most supplier quote documents have boilerplate, front or back, about things that are beyond their control, like supplier surcharges, design changes, all that stuff.
I like to develop long term relationships with vendors. Not involving reverse cash flows or favors or anything like that. I just tend to issue RFQs for non-commodity items to people with whom I have done business before, and had satisfactory results. That way, if a quote comes in higher than I expected, we can work together, over the phone or by email, to adjust the design so as to make everyone happy. Establishing that level of rapport, and the necessary common vocabulary, usually requires meeting in person at least once. That works better if it's not a social meeting, but a shop visit to hammer on an actual problem.
Pay the vendor. If it leaves a bad taste in your mouth, don't call him again. Maybe you could get lawyers involved, or cause him to do so, but you'd just end up paying lawyers, and being distracted from what you're really trying to do.
Mike Halloran
Pembroke Pines, FL, USA