Do you mean a contract or an invoice? One happens before the service is rendered and spells out the terms of the agreement (I assume you meant a contract), the other happens after the service is complete and bills them for services rendered. I'm not sure about the recording industry, but in my field, we usually require up front payment to cover our costs and then the final payment covers our fees and profit. That way you haven't technically lost anything if they screw you on either end.