Always put it in the contract. Otherwise, they can always come backwith a "No, you never told me that" or "That's not what you said before."
But make sure you have time limits on your contract. Otherwise, you may commit yourself to cheap support for a very long time. An annual contract (that is automatically renewed each year unless one or both party cancels) is pretty standard. When you plan to change your support rates in the future, send notice and a revised contract well ahaead of the renewal date.