The transaction should be mutual until the order of business is complete.
The full amount should not be settled until the final product is received to the satisfaction
Bargaining power stays with the person with the cash, the moment the cash changes hands it is almost impossible to bargain. Most of the time, unless the other person is concerned about attention to detail you would receive a bit unfavorable service.
Speak in fluent language with no scope for ambiguity - keep the agreement in plain terms and make sure both parties participate in the agreement and they are proactive (if possible ask them to repeat).