Apart from making every effort to ensure you have back up for equipment, batteries etc., in the event of their failure on the day, you can always offer to pre and or re shoot specificlly requested images. The important thing is to satisfy all parties concerned whatever the weather and circumstance.

Ensure verbal agreements are documented into written word. As in any business, there can always be disagreements, and that is why it is sensible to act proffesionally, even to make time and dated notes in a diary / note book of any telephone conversations.

Best of luck