Well, if you think a contract won't work for her, I would at least make a note of what you agreed to, print it up, date it, and sign it yourself. That way you know what was said, and she's less likely to try to snowball you.
Well - since this is how every human institution on the face of the earth works, I thinks it's ok to insist upon it. She may find it unpleasant, but heh, sometimes we find the realities of life unpleasant.
A contract is nothing but a visual reminder of what you and she have agreed to.