We changed repair materials, and the final bill now has an extra charge. How should change orders be handled?

Material substitutions, added work or newly discovered damage can legitimately change a repair price. Ask for a written change that identifies the affected work, additions and credits, price and schedule before approving it. Keep the agreement with the original contract, including changes said to cost nothing. If an extra charge appears only at the end, compare it with the agreed scope and messages, and ask the company to explain the calculation in writing. Insurance approval and your authorization are separate. Resolve disputed amounts with management; seek consumer-agency or legal advice when necessary.