For my parents paint and body shop they charge by hour. Different rates for body, paint, frame, and mechanical. There is a disclaimer at the bottom that says price is subject to change because of hidden damages. And there's a place where they sign. If its something small under $50 (jobs typically range from $500-3000) we don't usually contact them immediately! But if it's over $50 we usually call the customer and proceed from there! Hope this helps! Any questions you can email me! I could also send you an estimate spreadsheet if you'd like.