You want to get paid by your client. The Services Agreement should specify the amount that the client will pay you, both for the base services and for any additional work that may need to be performed. Many disagreements arise when more extensive work is required to achieve the client's goals. The Services Agreement should also specify when the client is going to pay you, whether that is within 15 days of receipt of their monthly bill or upon completion of the project.
Also, you should specify which out-of-pocket expenses, if any, will be reimbursed to you by the client.