We always work for a fixed price that will only be altered if there are change orders. The payment schedules vary dependent upon the amount of special-order products involved in your project, but typically we collect 15% of the contract sum at the time of signing the contract, 20% at the start of work, and then the remaining based on the amount of work completed at certain billing intervals (weekly, bi-weekly, etc.) There is always a balance that you hold on to until you are completely satisfied with out work.